bpo-32436: Implement PEP 567 (#5027)

41 files changed, 6269 insertions, 120 deletions

diff --git a/Include/Python.h b/Include/Python.h
index 20051e7ff0..dd595ea5e4 100644
--- a/Include/Python.h
+++ b/Include/Python.h
@@ -109,6 +109,7 @@
 #include "pyerrors.h"
 
 #include "pystate.h"
+#include "context.h"
 
 #include "pyarena.h"
 #include "modsupport.h"
diff --git a/Include/context.h b/Include/context.h
new file mode 100644
index 0000000000..f872dceee0
--- /dev/null
+++ b/Include/context.h
@@ -0,0 +1,86 @@
+#ifndef Py_CONTEXT_H
+#define Py_CONTEXT_H
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+#ifndef Py_LIMITED_API
+
+
+PyAPI_DATA(PyTypeObject) PyContext_Type;
+typedef struct _pycontextobject PyContext;
+
+PyAPI_DATA(PyTypeObject) PyContextVar_Type;
+typedef struct _pycontextvarobject PyContextVar;
+
+PyAPI_DATA(PyTypeObject) PyContextToken_Type;
+typedef struct _pycontexttokenobject PyContextToken;
+
+
+#define PyContext_CheckExact(o) (Py_TYPE(o) == &PyContext_Type)
+#define PyContextVar_CheckExact(o) (Py_TYPE(o) == &PyContextVar_Type)
+#define PyContextToken_CheckExact(o) (Py_TYPE(o) == &PyContextToken_Type)
+
+
+PyAPI_FUNC(PyContext *) PyContext_New(void);
+PyAPI_FUNC(PyContext *) PyContext_Copy(PyContext *);
+PyAPI_FUNC(PyContext *) PyContext_CopyCurrent(void);
+
+PyAPI_FUNC(int) PyContext_Enter(PyContext *);
+PyAPI_FUNC(int) PyContext_Exit(PyContext *);
+
+
+/* Create a new context variable.
+
+   default_value can be NULL.
+*/
+PyAPI_FUNC(PyContextVar *) PyContextVar_New(
+    const char *name, PyObject *default_value);
+
+
+/* Get a value for the variable.
+
+   Returns -1 if an error occurred during lookup.
+
+   Returns 0 if value either was or was not found.
+
+   If value was found, *value will point to it.
+   If not, it will point to:
+
+   - default_value, if not NULL;
+   - the default value of "var", if not NULL;
+   - NULL.
+
+   '*value' will be a new ref, if not NULL.
+*/
+PyAPI_FUNC(int) PyContextVar_Get(
+    PyContextVar *var, PyObject *default_value, PyObject **value);
+
+
+/* Set a new value for the variable.
+   Returns NULL if an error occurs.
+*/
+PyAPI_FUNC(PyContextToken *) PyContextVar_Set(
+    PyContextVar *var, PyObject *value);
+
+
+/* Reset a variable to its previous value.
+   Returns 0 on sucess, -1 on error.
+*/
+PyAPI_FUNC(int) PyContextVar_Reset(
+    PyContextVar *var, PyContextToken *token);
+
+
+/* This method is exposed only for CPython tests. Don not use it. */
+PyAPI_FUNC(PyObject *) _PyContext_NewHamtForTests(void);
+
+
+PyAPI_FUNC(int) PyContext_ClearFreeList(void);
+
+
+#endif /* !Py_LIMITED_API */
+
+#ifdef __cplusplus
+}
+#endif
+#endif /* !Py_CONTEXT_H */
diff --git a/Include/internal/context.h b/Include/internal/context.h
new file mode 100644
index 0000000000..59f88f2614
--- /dev/null
+++ b/Include/internal/context.h
@@ -0,0 +1,41 @@
+#ifndef Py_INTERNAL_CONTEXT_H
+#define Py_INTERNAL_CONTEXT_H
+
+
+#include "internal/hamt.h"
+
+
+struct _pycontextobject {
+    PyObject_HEAD
+    PyContext *ctx_prev;
+    PyHamtObject *ctx_vars;
+    PyObject *ctx_weakreflist;
+    int ctx_entered;
+};
+
+
+struct _pycontextvarobject {
+    PyObject_HEAD
+    PyObject *var_name;
+    PyObject *var_default;
+    PyObject *var_cached;
+    uint64_t var_cached_tsid;
+    uint64_t var_cached_tsver;
+    Py_hash_t var_hash;
+};
+
+
+struct _pycontexttokenobject {
+    PyObject_HEAD
+    PyContext *tok_ctx;
+    PyContextVar *tok_var;
+    PyObject *tok_oldval;
+    int tok_used;
+};
+
+
+int _PyContext_Init(void);
+void _PyContext_Fini(void);
+
+
+#endif /* !Py_INTERNAL_CONTEXT_H */
diff --git a/Include/internal/hamt.h b/Include/internal/hamt.h
new file mode 100644
index 0000000000..52488d0858
--- /dev/null
+++ b/Include/internal/hamt.h
@@ -0,0 +1,113 @@
+#ifndef Py_INTERNAL_HAMT_H
+#define Py_INTERNAL_HAMT_H
+
+
+#define _Py_HAMT_MAX_TREE_DEPTH 7
+
+
+#define PyHamt_Check(o) (Py_TYPE(o) == &_PyHamt_Type)
+
+
+/* Abstract tree node. */
+typedef struct {
+    PyObject_HEAD
+} PyHamtNode;
+
+
+/* An HAMT immutable mapping collection. */
+typedef struct {
+    PyObject_HEAD
+    PyHamtNode *h_root;
+    PyObject *h_weakreflist;
+    Py_ssize_t h_count;
+} PyHamtObject;
+
+
+/* A struct to hold the state of depth-first traverse of the tree.
+
+   HAMT is an immutable collection.  Iterators will hold a strong reference
+   to it, and every node in the HAMT has strong references to its children.
+
+   So for iterators, we can implement zero allocations and zero reference
+   inc/dec depth-first iteration.
+
+   - i_nodes: an array of seven pointers to tree nodes
+   - i_level: the current node in i_nodes
+   - i_pos: an array of positions within nodes in i_nodes.
+*/
+typedef struct {
+    PyHamtNode *i_nodes[_Py_HAMT_MAX_TREE_DEPTH];
+    Py_ssize_t i_pos[_Py_HAMT_MAX_TREE_DEPTH];
+    int8_t i_level;
+} PyHamtIteratorState;
+
+
+/* Base iterator object.
+
+   Contains the iteration state, a pointer to the HAMT tree,
+   and a pointer to the 'yield function'.  The latter is a simple
+   function that returns a key/value tuple for the 'Items' iterator,
+   just a key for the 'Keys' iterator, and a value for the 'Values'
+   iterator.
+*/
+typedef struct {
+    PyObject_HEAD
+    PyHamtObject *hi_obj;
+    PyHamtIteratorState hi_iter;
+    binaryfunc hi_yield;
+} PyHamtIterator;
+
+
+PyAPI_DATA(PyTypeObject) _PyHamt_Type;
+PyAPI_DATA(PyTypeObject) _PyHamt_ArrayNode_Type;
+PyAPI_DATA(PyTypeObject) _PyHamt_BitmapNode_Type;
+PyAPI_DATA(PyTypeObject) _PyHamt_CollisionNode_Type;
+PyAPI_DATA(PyTypeObject) _PyHamtKeys_Type;
+PyAPI_DATA(PyTypeObject) _PyHamtValues_Type;
+PyAPI_DATA(PyTypeObject) _PyHamtItems_Type;
+
+
+/* Create a new HAMT immutable mapping. */
+PyHamtObject * _PyHamt_New(void);
+
+/* Return a new collection based on "o", but with an additional
+   key/val pair. */
+PyHamtObject * _PyHamt_Assoc(PyHamtObject *o, PyObject *key, PyObject *val);
+
+/* Return a new collection based on "o", but without "key". */
+PyHamtObject * _PyHamt_Without(PyHamtObject *o, PyObject *key);
+
+/* Find "key" in the "o" collection.
+
+   Return:
+   - -1: An error ocurred.
+   - 0: "key" wasn't found in "o".
+   - 1: "key" is in "o"; "*val" is set to its value (a borrowed ref).
+*/
+int _PyHamt_Find(PyHamtObject *o, PyObject *key, PyObject **val);
+
+/* Check if "v" is equal to "w".
+
+   Return:
+   - 0: v != w
+   - 1: v == w
+   - -1: An error occurred.
+*/
+int _PyHamt_Eq(PyHamtObject *v, PyHamtObject *w);
+
+/* Return the size of "o"; equivalent of "len(o)". */
+Py_ssize_t _PyHamt_Len(PyHamtObject *o);
+
+/* Return a Keys iterator over "o". */
+PyObject * _PyHamt_NewIterKeys(PyHamtObject *o);
+
+/* Return a Values iterator over "o". */
+PyObject * _PyHamt_NewIterValues(PyHamtObject *o);
+
+/* Return a Items iterator over "o". */
+PyObject * _PyHamt_NewIterItems(PyHamtObject *o);
+
+int _PyHamt_Init(void);
+void _PyHamt_Fini(void);
+
+#endif /* !Py_INTERNAL_HAMT_H */
diff --git a/Include/pystate.h b/Include/pystate.h
index 5a69e1471a..d004be5e90 100644
--- a/Include/pystate.h
+++ b/Include/pystate.h
@@ -143,6 +143,8 @@ typedef struct _is {
     /* AtExit module */
     void (*pyexitfunc)(PyObject *);
     PyObject *pyexitmodule;
+
+    uint64_t tstate_next_unique_id;
 } PyInterpreterState;
 #endif   /* !Py_LIMITED_API */
 
@@ -270,6 +272,12 @@ typedef struct _ts {
     PyObject *async_gen_firstiter;
     PyObject *async_gen_finalizer;
 
+    PyObject *context;
+    uint64_t context_ver;
+
+    /* Unique thread state id. */
+    uint64_t id;
+
     /* XXX signal handlers should also be here */
 
 } PyThreadState;
diff --git a/Lib/asyncio/base_events.py b/Lib/asyncio/base_events.py
index ca9eee765e..e722cf26b5 100644
--- a/Lib/asyncio/base_events.py
+++ b/Lib/asyncio/base_events.py
@@ -489,7 +489,7 @@ class BaseEventLoop(events.AbstractEventLoop):
         """
         return time.monotonic()
 
-    def call_later(self, delay, callback, *args):
+    def call_later(self, delay, callback, *args, context=None):
         """Arrange for a callback to be called at a given time.
 
         Return a Handle: an opaque object with a cancel() method that
@@ -505,12 +505,13 @@ class BaseEventLoop(events.AbstractEventLoop):
         Any positional arguments after the callback will be passed to
         the callback when it is called.
         """
-        timer = self.call_at(self.time() + delay, callback, *args)
+        timer = self.call_at(self.time() + delay, callback, *args,
+                             context=context)
         if timer._source_traceback:
             del timer._source_traceback[-1]
         return timer
 
-    def call_at(self, when, callback, *args):
+    def call_at(self, when, callback, *args, context=None):
         """Like call_later(), but uses an absolute time.
 
         Absolute time corresponds to the event loop's time() method.
@@ -519,14 +520,14 @@ class BaseEventLoop(events.AbstractEventLoop):
         if self._debug:
             self._check_thread()
             self._check_callback(callback, 'call_at')
-        timer = events.TimerHandle(when, callback, args, self)
+        timer = events.TimerHandle(when, callback, args, self, context)
         if timer._source_traceback:
             del timer._source_traceback[-1]
         heapq.heappush(self._scheduled, timer)
         timer._scheduled = True
         return timer
 
-    def call_soon(self, callback, *args):
+    def call_soon(self, callback, *args, context=None):
         """Arrange for a callback to be called as soon as possible.
 
         This operates as a FIFO queue: callbacks are called in the
@@ -540,7 +541,7 @@ class BaseEventLoop(events.AbstractEventLoop):
         if self._debug:
             self._check_thread()
             self._check_callback(callback, 'call_soon')
-        handle = self._call_soon(callback, args)
+        handle = self._call_soon(callback, args, context)
         if handle._source_traceback:
             del handle._source_traceback[-1]
         return handle
@@ -555,8 +556,8 @@ class BaseEventLoop(events.AbstractEventLoop):
                 f'a callable object was expected by {method}(), '
                 f'got {callback!r}')
 
-    def _call_soon(self, callback, args):
-        handle = events.Handle(callback, args, self)
+    def _call_soon(self, callback, args, context):
+        handle = events.Handle(callback, args, self, context)
         if handle._source_traceback:
             del handle._source_traceback[-1]
         self._ready.append(handle)
@@ -579,12 +580,12 @@ class BaseEventLoop(events.AbstractEventLoop):
                 "Non-thread-safe operation invoked on an event loop other "
                 "than the current one")
 
-    def call_soon_threadsafe(self, callback, *args):
+    def call_soon_threadsafe(self, callback, *args, context=None):
         """Like call_soon(), but thread-safe."""
         self._check_closed()
         if self._debug:
             self._check_callback(callback, 'call_soon_threadsafe')
-        handle = self._call_soon(callback, args)
+        handle = self._call_soon(callback, args, context)
         if handle._source_traceback:
             del handle._source_traceback[-1]
         self._write_to_self()
diff --git a/Lib/asyncio/base_futures.py b/Lib/asyncio/base_futures.py
index 008812eda9..5182884e16 100644
--- a/Lib/asyncio/base_futures.py
+++ b/Lib/asyncio/base_futures.py
@@ -41,13 +41,13 @@ def _format_callbacks(cb):
         return format_helpers._format_callback_source(callback, ())
 
     if size == 1:
-        cb = format_cb(cb[0])
+        cb = format_cb(cb[0][0])
     elif size == 2:
-        cb = '{}, {}'.format(format_cb(cb[0]), format_cb(cb[1]))
+        cb = '{}, {}'.format(format_cb(cb[0][0]), format_cb(cb[1][0]))
     elif size > 2:
-        cb = '{}, <{} more>, {}'.format(format_cb(cb[0]),
+        cb = '{}, <{} more>, {}'.format(format_cb(cb[0][0]),
                                         size - 2,
-                                        format_cb(cb[-1]))
+                                        format_cb(cb[-1][0]))
     return f'cb=[{cb}]'
 
 
diff --git a/Lib/asyncio/events.py b/Lib/asyncio/events.py
index d5365dc480..5c68d4cb97 100644
--- a/Lib/asyncio/events.py
+++ b/Lib/asyncio/events.py
@@ -11,6 +11,7 @@ __all__ = (
     '_get_running_loop',
 )
 
+import contextvars
 import os
 import socket
 import subprocess
@@ -32,9 +33,13 @@ class Handle:
     """Object returned by callback registration methods."""
 
     __slots__ = ('_callback', '_args', '_cancelled', '_loop',
-                 '_source_traceback', '_repr', '__weakref__')
+                 '_source_traceback', '_repr', '__weakref__',
+                 '_context')
 
-    def __init__(self, callback, args, loop):
+    def __init__(self, callback, args, loop, context=None):
+        if context is None:
+            context = contextvars.copy_context()
+        self._context = context
         self._loop = loop
         self._callback = callback
         self._args = args
@@ -80,7 +85,7 @@ class Handle:
 
     def _run(self):
         try:
-            self._callback(*self._args)
+            self._context.run(self._callback, *self._args)
         except Exception as exc:
             cb = format_helpers._format_callback_source(
                 self._callback, self._args)
@@ -101,9 +106,9 @@ class TimerHandle(Handle):
 
     __slots__ = ['_scheduled', '_when']
 
-    def __init__(self, when, callback, args, loop):
+    def __init__(self, when, callback, args, loop, context=None):
         assert when is not None
-        super().__init__(callback, args, loop)
+        super().__init__(callback, args, loop, context)
         if self._source_traceback:
             del self._source_traceback[-1]
         self._when = when
diff --git a/Lib/asyncio/futures.py b/Lib/asyncio/futures.py
index 1c05b2231c..59621ffb6e 100644
--- a/Lib/asyncio/futures.py
+++ b/Lib/asyncio/futures.py
@@ -6,6 +6,7 @@ __all__ = (
 )
 
 import concurrent.futures
+import contextvars
 import logging
 import sys
 
@@ -144,8 +145,8 @@ class Future:
             return
 
         self._callbacks[:] = []
-        for callback in callbacks:
-            self._loop.call_soon(callback, self)
+        for callback, ctx in callbacks:
+            self._loop.call_soon(callback, self, context=ctx)
 
     def cancelled(self):
         """Return True if the future was cancelled."""
@@ -192,7 +193,7 @@ class Future:
         self.__log_traceback = False
         return self._exception
 
-    def add_done_callback(self, fn):
+    def add_done_callback(self, fn, *, context=None):
         """Add a callback to be run when the future becomes done.
 
         The callback is called with a single argument - the future object. If
@@ -200,9 +201,11 @@ class Future:
         scheduled with call_soon.
         """
         if self._state != _PENDING:
-            self._loop.call_soon(fn, self)
+            self._loop.call_soon(fn, self, context=context)
         else:
-            self._callbacks.append(fn)
+            if context is None:
+                context = contextvars.copy_context()
+            self._callbacks.append((fn, context))
 
     # New method not in PEP 3148.
 
@@ -211,7 +214,9 @@ class Future:
 
         Returns the number of callbacks removed.
         """
-        filtered_callbacks = [f for f in self._callbacks if f != fn]
+        filtered_callbacks = [(f, ctx)
+                              for (f, ctx) in self._callbacks
+                              if f != fn]
         removed_count = len(self._callbacks) - len(filtered_callbacks)
         if removed_count:
             self._callbacks[:] = filtered_callbacks
diff --git a/Lib/asyncio/selector_events.py b/Lib/asyncio/selector_events.py
index 5692e38486..9446ae6a3b 100644
--- a/Lib/asyncio/selector_events.py
+++ b/Lib/asyncio/selector_events.py
@@ -256,7 +256,7 @@ class BaseSelectorEventLoop(base_events.BaseEventLoop):
 
     def _add_reader(self, fd, callback, *args):
         self._check_closed()
-        handle = events.Handle(callback, args, self)
+        handle = events.Handle(callback, args, self, None)
         try:
             key = self._selector.get_key(fd)
         except KeyError:
@@ -292,7 +292,7 @@ class BaseSelectorEventLoop(base_events.BaseEventLoop):
 
     def _add_writer(self, fd, callback, *args):
         self._check_closed()
-        handle = events.Handle(callback, args, self)
+        handle = events.Handle(callback, args, self, None)
         try:
             key = self._selector.get_key(fd)
         except KeyError:
diff --git a/Lib/asyncio/tasks.py b/Lib/asyncio/tasks.py
index b11808853e..609b8e8a04 100644
--- a/Lib/asyncio/tasks.py
+++ b/Lib/asyncio/tasks.py
@@ -10,6 +10,7 @@ __all__ = (
 )
 
 import concurrent.futures
+import contextvars
 import functools
 import inspect
 import types
@@ -96,8 +97,9 @@ class Task(futures._PyFuture):  # Inherit Python Task implementation
         self._must_cancel = False
         self._fut_waiter = None
         self._coro = coro
+        self._context = contextvars.copy_context()
 
-        self._loop.call_soon(self._step)
+        self._loop.call_soon(self._step, context=self._context)
         _register_task(self)
 
     def __del__(self):
@@ -229,15 +231,18 @@ class Task(futures._PyFuture):  # Inherit Python Task implementation
                     new_exc = RuntimeError(
                         f'Task {self!r} got Future '
                         f'{result!r} attached to a different loop')
-                    self._loop.call_soon(self._step, new_exc)
+                    self._loop.call_soon(
+                        self._step, new_exc, context=self._context)
                 elif blocking:
                     if result is self:
                         new_exc = RuntimeError(
                             f'Task cannot await on itself: {self!r}')
-                        self._loop.call_soon(self._step, new_exc)
+                        self._loop.call_soon(
+                            self._step, new_exc, context=self._context)
                     else:
                         result._asyncio_future_blocking = False
-                        result.add_done_callback(self._wakeup)
+                        result.add_done_callback(
+                            self._wakeup, context=self._context)
                         self._fut_waiter = result
                         if self._must_cancel:
                             if self._fut_waiter.cancel():
@@ -246,21 +251,24 @@ class Task(futures._PyFuture):  # Inherit Python Task implementation
                     new_exc = RuntimeError(
                         f'yield was used instead of yield from '
                         f'in task {self!r} with {result!r}')
-                    self._loop.call_soon(self._step, new_exc)
+                    self._loop.call_soon(
+                        self._step, new_exc, context=self._context)
 
             elif result is None:
                 # Bare yield relinquishes control for one event loop iteration.
-                self._loop.call_soon(self._step)
+                self._loop.call_soon(self._step, context=self._context)
             elif inspect.isgenerator(result):
                 # Yielding a generator is just wrong.
                 new_exc = RuntimeError(
                     f'yield was used instead of yield from for '
                     f'generator in task {self!r} with {result}')
-                self._loop.call_soon(self._step, new_exc)
+                self._loop.call_soon(
+                    self._step, new_exc, context=self._context)
             else:
                 # Yielding something else is an error.
                 new_exc = RuntimeError(f'Task got bad yield: {result!r}')
-                self._loop.call_soon(self._step, new_exc)
+                self._loop.call_soon(
+                    self._step, new_exc, context=self._context)
         finally:
             _leave_task(self._loop, self)
             self = None  # Needed to break cycles when an exception occurs.
diff --git a/Lib/asyncio/unix_events.py b/Lib/asyncio/unix_events.py
index 028a0ca8f8..9b9d0043b5 100644
--- a/Lib/asyncio/unix_events.py
+++ b/Lib/asyncio/unix_events.py
@@ -92,7 +92,7 @@ class _UnixSelectorEventLoop(selector_events.BaseSelectorEventLoop):
         except (ValueError, OSError) as exc:
             raise RuntimeError(str(exc))
 
-        handle = events.Handle(callback, args, self)
+        handle = events.Handle(callback, args, self, None)
         self._signal_handlers[sig] = handle
 
         try:
diff --git a/Lib/contextvars.py b/Lib/contextvars.py
new file mode 100644
index 0000000000..d78c80dfe6
--- /dev/null
+++ b/Lib/contextvars.py
@@ -0,0 +1,4 @@
+from _contextvars import Context, ContextVar, Token, copy_context
+
+
+__all__ = ('Context', 'ContextVar', 'Token', 'copy_context')
diff --git a/Lib/test/test_asyncio/test_base_events.py b/Lib/test/test_asyncio/test_base_events.py
index fc3b81096d..8d72df6a72 100644
--- a/Lib/test/test_asyncio/test_base_events.py
+++ b/Lib/test/test_asyncio/test_base_events.py
@@ -192,14 +192,14 @@ class BaseEventLoopTests(test_utils.TestCase):
         self.assertRaises(RuntimeError, self.loop.run_until_complete, f)
 
     def test__add_callback_handle(self):
-        h = asyncio.Handle(lambda: False, (), self.loop)
+        h = asyncio.Handle(lambda: False, (), self.loop, None)
 
         self.loop._add_callback(h)
         self.assertFalse(self.loop._scheduled)
         self.assertIn(h, self.loop._ready)
 
     def test__add_callback_cancelled_handle(self):
-        h = asyncio.Handle(lambda: False, (), self.loop)
+        h = asyncio.Handle(lambda: False, (), self.loop, None)
         h.cancel()
 
         self.loop._add_callback(h)
@@ -333,9 +333,9 @@ class BaseEventLoopTests(test_utils.TestCase):
 
     def test__run_once(self):
         h1 = asyncio.TimerHandle(time.monotonic() + 5.0, lambda: True, (),
-                                 self.loop)
+                                 self.loop, None)
         h2 = asyncio.TimerHandle(time.monotonic() + 10.0, lambda: True, (),
-                                 self.loop)
+                                 self.loop, None)
 
         h1.cancel()
 
@@ -390,7 +390,7 @@ class BaseEventLoopTests(test_utils.TestCase):
             handle = loop.call_soon(lambda: True)
 
         h = asyncio.TimerHandle(time.monotonic() - 1, cb, (self.loop,),
-                                self.loop)
+                                self.loop, None)
 
         self.loop._process_events = mock.Mock()
         self.loop._scheduled.append(h)
diff --git a/Lib/test/test_asyncio/test_futures.py b/Lib/test/test_asyncio/test_futures.py
index ab45ee39ab..37f4c6562f 100644
--- a/Lib/test/test_asyncio/test_futures.py
+++ b/Lib/test/test_asyncio/test_futures.py
@@ -565,16 +565,22 @@ class BaseFutureTests:
 @unittest.skipUnless(hasattr(futures, '_CFuture'),
                      'requires the C _asyncio module')
 class CFutureTests(BaseFutureTests, test_utils.TestCase):
-    cls = futures._CFuture
+    try:
+        cls = futures._CFuture
+    except AttributeError:
+        cls = None
 
 
 @unittest.skipUnless(hasattr(futures, '_CFuture'),
                      'requires the C _asyncio module')
 class CSubFutureTests(BaseFutureTests, test_utils.TestCase):
-    class CSubFuture(futures._CFuture):
-        pass
+    try:
+        class CSubFuture(futures._CFuture):
+            pass
 
-    cls = CSubFuture
+        cls = CSubFuture
+    except AttributeError:
+        cls = None
 
 
 class PyFutureTests(BaseFutureTests, test_utils.TestCase):
diff --git a/Lib/test/test_asyncio/test_tasks.py b/Lib/test/test_asyncio/test_tasks.py
index 26e4f643d1..96d2658cb4 100644
--- a/Lib/test/test_asyncio/test_tasks.py
+++ b/Lib/test/test_asyncio/test_tasks.py
@@ -2,10 +2,11 @@
 
 import collections
 import contextlib
+import contextvars
 import functools
 import gc
 import io
-import os
+import random
 import re
 import sys
 import types
@@ -1377,9 +1378,9 @@ class BaseTaskTests:
                 self.cb_added = False
                 super().__init__(*args, **kwds)
 
-            def add_done_callback(self, fn):
+            def add_done_callback(self, *args, **kwargs):
                 self.cb_added = True
-                super().add_done_callback(fn)
+                super().add_done_callback(*args, **kwargs)
 
         fut = Fut(loop=self.loop)
         result = None
@@ -2091,7 +2092,7 @@ class BaseTaskTests:
 
     @mock.patch('asyncio.base_events.logger')
     def test_error_in_call_soon(self, m_log):
-        def call_soon(callback, *args):
+        def call_soon(callback, *args, **kwargs):
             raise ValueError
         self.loop.call_soon = call_soon
 
@@ -2176,6 +2177,91 @@ class BaseTaskTests:
 
         self.loop.run_until_complete(coro())
 
+    def test_context_1(self):
+        cvar = contextvars.ContextVar('cvar', default='nope')
+
+        async def sub():
+            await asyncio.sleep(0.01, loop=loop)
+            self.assertEqual(cvar.get(), 'nope')
+            cvar.set('something else')
+
+        async def main():
+            self.assertEqual(cvar.get(), 'nope')
+            subtask = self.new_task(loop, sub())
+            cvar.set('yes')
+            self.assertEqual(cvar.get(), 'yes')
+            await subtask
+            self.assertEqual(cvar.get(), 'yes')
+
+        loop = asyncio.new_event_loop()
+        try:
+            task = self.new_task(loop, main())
+            loop.run_until_complete(task)
+        finally:
+            loop.close()
+
+    def test_context_2(self):
+        cvar = contextvars.ContextVar('cvar', default='nope')
+
+        async def main():
+            def fut_on_done(fut):
+                # This change must not pollute the context
+                # of the "main()" task.
+                cvar.set('something else')
+
+            self.assertEqual(cvar.get(), 'nope')
+
+            for j in range(2):
+                fut = self.new_future(loop)
+                fut.add_done_callback(fut_on_done)
+                cvar.set(f'yes{j}')
+                loop.call_soon(fut.set_result, None)
+                await fut
+                self.assertEqual(cvar.get(), f'yes{j}')
+
+                for i in range(3):
+                    # Test that task passed its context to add_done_callback:
+                    cvar.set(f'yes{i}-{j}')
+                    await asyncio.sleep(0.001, loop=loop)
+                    self.assertEqual(cvar.get(), f'yes{i}-{j}')
+
+        loop = asyncio.new_event_loop()
+        try:
+            task = self.new_task(loop, main())
+            loop.run_until_complete(task)
+        finally:
+            loop.close()
+
+        self.assertEqual(cvar.get(), 'nope')
+
+    def test_context_3(self):
+        # Run 100 Tasks in parallel, each modifying cvar.
+
+        cvar = contextvars.ContextVar('cvar', default=-1)
+
+        async def sub(num):
+            for i in range(10):
+                cvar.set(num + i)
+                await asyncio.sleep(
+                    random.uniform(0.001, 0.05), loop=loop)
+                self.assertEqual(cvar.get(), num + i)
+
+        async def main():
+            tasks = []
+            for i in range(100):
+                task = loop.create_task(sub(random.randint(0, 10)))
+                tasks.append(task)
+
+            await asyncio.gather(*tasks, loop=loop)
+
+        loop = asyncio.new_event_loop()
+        try:
+            loop.run_until_complete(main())
+        finally:
+            loop.close()
+
+        self.assertEqual(cvar.get(), -1)
+
 
 def add_subclass_tests(cls):
     BaseTask = cls.Task
@@ -2193,9 +2279,9 @@ def add_subclass_tests(cls):
             self.calls['_schedule_callbacks'] += 1
             return super()._schedule_callbacks()
 
-        def add_done_callback(self, *args):
+        def add_done_callback(self, *args, **kwargs):
             self.calls['add_done_callback'] += 1
-            return super().add_done_callback(*args)
+            return super().add_done_callback(*args, **kwargs)
 
     class Task(CommonFuture, BaseTask):
         def _step(self, *args):
@@ -2486,10 +2572,13 @@ class PyIntrospectionTests(unittest.TestCase, BaseTaskIntrospectionTests):
 @unittest.skipUnless(hasattr(tasks, '_c_register_task'),
                      'requires the C _asyncio module')
 class CIntrospectionTests(unittest.TestCase, BaseTaskIntrospectionTests):
-    _register_task = staticmethod(tasks._c_register_task)
-    _unregister_task = staticmethod(tasks._c_unregister_task)
-    _enter_task = staticmethod(tasks._c_enter_task)
-    _leave_task = staticmethod(tasks._c_leave_task)
+    if hasattr(tasks, '_c_register_task'):
+        _register_task = staticmethod(tasks._c_register_task)
+        _unregister_task = staticmethod(tasks._c_unregister_task)
+        _enter_task = staticmethod(tasks._c_enter_task)
+        _leave_task = staticmethod(tasks._c_leave_task)
+    else:
+        _register_task = _unregister_task = _enter_task = _leave_task = None
 
 
 class BaseCurrentLoopTests:
diff --git a/Lib/test/test_asyncio/utils.py b/Lib/test/test_asyncio/utils.py
index f756ec9016..96dfe2f85b 100644
--- a/Lib/test/test_asyncio/utils.py
+++ b/Lib/test/test_asyncio/utils.py
@@ -365,7 +365,7 @@ class TestLoop(base_events.BaseEventLoop):
                 raise AssertionError("Time generator is not finished")
 
     def _add_reader(self, fd, callback, *args):
-        self.readers[fd] = events.Handle(callback, args, self)
+        self.readers[fd] = events.Handle(callback, args, self, None)
 
     def _remove_reader(self, fd):
         self.remove_reader_count[fd] += 1
@@ -391,7 +391,7 @@ class TestLoop(base_events.BaseEventLoop):
             raise AssertionError(f'fd {fd} is registered')
 
     def _add_writer(self, fd, callback, *args):
-        self.writers[fd] = events.Handle(callback, args, self)
+        self.writers[fd] = events.Handle(callback, args, self, None)
 
     def _remove_writer(self, fd):
         self.remove_writer_count[fd] += 1
@@ -457,9 +457,9 @@ class TestLoop(base_events.BaseEventLoop):
             self.advance_time(advance)
         self._timers = []
 
-    def call_at(self, when, callback, *args):
+    def call_at(self, when, callback, *args, context=None):
         self._timers.append(when)
-        return super().call_at(when, callback, *args)
+        return super().call_at(when, callback, *args, context=context)
 
     def _process_events(self, event_list):
         return
diff --git a/Lib/test/test_context.py b/Lib/test/test_context.py
new file mode 100644
index 0000000000..74d05fc5c4
--- /dev/null
+++ b/Lib/test/test_context.py
@@ -0,0 +1,1064 @@
+import concurrent.futures
+import contextvars
+import functools
+import gc
+import random
+import time
+import unittest
+import weakref
+
+try:
+    from _testcapi import hamt
+except ImportError:
+    hamt = None
+
+
+def isolated_context(func):
+    """Needed to make reftracking test mode work."""
+    @functools.wraps(func)
+    def wrapper(*args, **kwargs):
+        ctx = contextvars.Context()
+        return ctx.run(func, *args, **kwargs)
+    return wrapper
+
+
+class ContextTest(unittest.TestCase):
+    def test_context_var_new_1(self):
+        with self.assertRaisesRegex(TypeError, 'takes exactly 1'):
+            contextvars.ContextVar()
+
+        with self.assertRaisesRegex(TypeError, 'must be a str'):
+            contextvars.ContextVar(1)
+
+        c = contextvars.ContextVar('a')
+        self.assertNotEqual(hash(c), hash('a'))
+
+    def test_context_var_new_2(self):
+        self.assertIsNone(contextvars.ContextVar[int])
+
+    @isolated_context
+    def test_context_var_repr_1(self):
+        c = contextvars.ContextVar('a')
+        self.assertIn('a', repr(c))
+
+        c = contextvars.ContextVar('a', default=123)
+        self.assertIn('123', repr(c))
+
+        lst = []
+        c = contextvars.ContextVar('a', default=lst)
+        lst.append(c)
+        self.assertIn('...', repr(c))
+        self.assertIn('...', repr(lst))
+
+        t = c.set(1)
+        self.assertIn(repr(c), repr(t))
+        self.assertNotIn(' used ', repr(t))
+        c.reset(t)
+        self.assertIn(' used ', repr(t))
+
+    def test_context_subclassing_1(self):
+        with self.assertRaisesRegex(TypeError, 'not an acceptable base type'):
+            class MyContextVar(contextvars.ContextVar):
+                # Potentially we might want ContextVars to be subclassable.
+                pass
+
+        with self.assertRaisesRegex(TypeError, 'not an acceptable base type'):
+            class MyContext(contextvars.Context):
+                pass
+
+        with self.assertRaisesRegex(TypeError, 'not an acceptable base type'):
+            class MyToken(contextvars.Token):
+                pass
+
+    def test_context_new_1(self):
+        with self.assertRaisesRegex(TypeError, 'any arguments'):
+            contextvars.Context(1)
+        with self.assertRaisesRegex(TypeError, 'any arguments'):
+            contextvars.Context(1, a=1)
+        with self.assertRaisesRegex(TypeError, 'any arguments'):
+            contextvars.Context(a=1)
+        contextvars.Context(**{})
+
+    def test_context_typerrors_1(self):
+        ctx = contextvars.Context()
+
+        with self.assertRaisesRegex(TypeError, 'ContextVar key was expected'):
+            ctx[1]
+        with self.assertRaisesRegex(TypeError, 'ContextVar key was expected'):
+            1 in ctx
+        with self.assertRaisesRegex(TypeError, 'ContextVar key was expected'):
+            ctx.get(1)
+
+    def test_context_get_context_1(self):
+        ctx = contextvars.copy_context()
+        self.assertIsInstance(ctx, contextvars.Context)
+
+    def test_context_run_1(self):
+        ctx = contextvars.Context()
+
+        with self.assertRaisesRegex(TypeError, 'missing 1 required'):
+            ctx.run()
+
+    def test_context_run_2(self):
+        ctx = contextvars.Context()
+
+        def func(*args, **kwargs):
+            kwargs['spam'] = 'foo'
+            args += ('bar',)
+            return args, kwargs
+
+        for f in (func, functools.partial(func)):
+            # partial doesn't support FASTCALL
+
+            self.assertEqual(ctx.run(f), (('bar',), {'spam': 'foo'}))
+            self.assertEqual(ctx.run(f, 1), ((1, 'bar'), {'spam': 'foo'}))
+
+            self.assertEqual(
+                ctx.run(f, a=2),
+                (('bar',), {'a': 2, 'spam': 'foo'}))
+
+            self.assertEqual(
+                ctx.run(f, 11, a=2),
+                ((11, 'bar'), {'a': 2, 'spam': 'foo'}))
+
+            a = {}
+            self.assertEqual(
+                ctx.run(f, 11, **a),
+                ((11, 'bar'), {'spam': 'foo'}))
+            self.assertEqual(a, {})
+
+    def test_context_run_3(self):
+        ctx = contextvars.Context()
+
+        def func(*args, **kwargs):
+            1 / 0
+
+        with self.assertRaises(ZeroDivisionError):
+            ctx.run(func)
+        with self.assertRaises(ZeroDivisionError):
+            ctx.run(func, 1, 2)
+        with self.assertRaises(ZeroDivisionError):
+            ctx.run(func, 1, 2, a=123)
+
+    @isolated_context
+    def test_context_run_4(self):
+        ctx1 = contextvars.Context()
+        ctx2 = contextvars.Context()
+        var = contextvars.ContextVar('var')
+
+        def func2():
+            self.assertIsNone(var.get(None))
+
+        def func1():
+            self.assertIsNone(var.get(None))
+            var.set('spam')
+            ctx2.run(func2)
+            self.assertEqual(var.get(None), 'spam')
+
+            cur = contextvars.copy_context()
+            self.assertEqual(len(cur), 1)
+            self.assertEqual(cur[var], 'spam')
+            return cur
+
+        returned_ctx = ctx1.run(func1)
+        self.assertEqual(ctx1, returned_ctx)
+        self.assertEqual(returned_ctx[var], 'spam')
+        self.assertIn(var, returned_ctx)
+
+    def test_context_run_5(self):
+        ctx = contextvars.Context()
+        var = contextvars.ContextVar('var')
+
+        def func():
+            self.assertIsNone(var.get(None))
+            var.set('spam')
+            1 / 0
+
+        with self.assertRaises(ZeroDivisionError):
+            ctx.run(func)
+
+        self.assertIsNone(var.get(None))
+
+    def test_context_run_6(self):
+        ctx = contextvars.Context()
+        c = contextvars.ContextVar('a', default=0)
+
+        def fun():
+            self.assertEqual(c.get(), 0)
+            self.assertIsNone(ctx.get(c))
+
+            c.set(42)
+            self.assertEqual(c.get(), 42)
+            self.assertEqual(ctx.get(c), 42)
+
+        ctx.run(fun)
+
+    def test_context_run_7(self):
+        ctx = contextvars.Context()
+
+        def fun():
+            with self.assertRaisesRegex(RuntimeError, 'is already entered'):
+                ctx.run(fun)
+
+        ctx.run(fun)
+
+    @isolated_context
+    def test_context_getset_1(self):
+        c = contextvars.ContextVar('c')
+        with self.assertRaises(LookupError):
+            c.get()
+
+        self.assertIsNone(c.get(None))
+
+        t0 = c.set(42)
+        self.assertEqual(c.get(), 42)
+        self.assertEqual(c.get(None), 42)
+        self.assertIs(t0.old_value, t0.MISSING)
+        self.assertIs(t0.old_value, contextvars.Token.MISSING)
+        self.assertIs(t0.var, c)
+
+        t = c.set('spam')
+        self.assertEqual(c.get(), 'spam')
+        self.assertEqual(c.get(None), 'spam')
+        self.assertEqual(t.old_value, 42)
+        c.reset(t)
+
+        self.assertEqual(c.get(), 42)
+        self.assertEqual(c.get(None), 42)
+
+        c.set('spam2')
+        with self.assertRaisesRegex(RuntimeError, 'has already been used'):
+            c.reset(t)
+        self.assertEqual(c.get(), 'spam2')
+
+        ctx1 = contextvars.copy_context()
+        self.assertIn(c, ctx1)
+
+        c.reset(t0)
+        with self.assertRaisesRegex(RuntimeError, 'has already been used'):
+            c.reset(t0)
+        self.assertIsNone(c.get(None))
+
+        self.assertIn(c, ctx1)
+        self.assertEqual(ctx1[c], 'spam2')
+        self.assertEqual(ctx1.get(c, 'aa'), 'spam2')
+        self.assertEqual(len(ctx1), 1)
+        self.assertEqual(list(ctx1.items()), [(c, 'spam2')])
+        self.assertEqual(list(ctx1.values()), ['spam2'])
+        self.assertEqual(list(ctx1.keys()), [c])
+        self.assertEqual(list(ctx1), [c])
+
+        ctx2 = contextvars.copy_context()
+        self.assertNotIn(c, ctx2)
+        with self.assertRaises(KeyError):
+            ctx2[c]
+        self.assertEqual(ctx2.get(c, 'aa'), 'aa')
+        self.assertEqual(len(ctx2), 0)
+        self.assertEqual(list(ctx2), [])
+
+    @isolated_context
+    def test_context_getset_2(self):
+        v1 = contextvars.ContextVar('v1')
+        v2 = contextvars.ContextVar('v2')
+
+        t1 = v1.set(42)
+        with self.assertRaisesRegex(ValueError, 'by a different'):
+            v2.reset(t1)
+
+    @isolated_context
+    def test_context_getset_3(self):
+        c = contextvars.ContextVar('c', default=42)
+        ctx = contextvars.Context()
+
+        def fun():
+            self.assertEqual(c.get(), 42)
+            with self.assertRaises(KeyError):
+                ctx[c]
+            self.assertIsNone(ctx.get(c))
+            self.assertEqual(ctx.get(c, 'spam'), 'spam')
+            self.assertNotIn(c, ctx)
+            self.assertEqual(list(ctx.keys()), [])
+
+            t = c.set(1)
+            self.assertEqual(list(ctx.keys()), [c])
+            self.assertEqual(ctx[c], 1)
+
+            c.reset(t)
+            self.assertEqual(list(ctx.keys()), [])
+            with self.assertRaises(KeyError):
+                ctx[c]
+
+        ctx.run(fun)
+
+    @isolated_context
+    def test_context_getset_4(self):
+        c = contextvars.ContextVar('c', default=42)
+        ctx = contextvars.Context()
+
+        tok = ctx.run(c.set, 1)
+
+        with self.assertRaisesRegex(ValueError, 'different Context'):
+            c.reset(tok)
+
+    @isolated_context
+    def test_context_getset_5(self):
+        c = contextvars.ContextVar('c', default=42)
+        c.set([])
+
+        def fun():
+            c.set([])
+            c.get().append(42)
+            self.assertEqual(c.get(), [42])
+
+        contextvars.copy_context().run(fun)
+        self.assertEqual(c.get(), [])
+
+    def test_context_copy_1(self):
+        ctx1 = contextvars.Context()
+        c = contextvars.ContextVar('c', default=42)
+
+        def ctx1_fun():
+            c.set(10)
+
+            ctx2 = ctx1.copy()
+            self.assertEqual(ctx2[c], 10)
+
+            c.set(20)
+            self.assertEqual(ctx1[c], 20)
+            self.assertEqual(ctx2[c], 10)
+
+            ctx2.run(ctx2_fun)
+            self.assertEqual(ctx1[c], 20)
+            self.assertEqual(ctx2[c], 30)
+
+        def ctx2_fun():
+            self.assertEqual(c.get(), 10)
+            c.set(30)
+            self.assertEqual(c.get(), 30)
+
+        ctx1.run(ctx1_fun)
+
+    @isolated_context
+    def test_context_threads_1(self):
+        cvar = contextvars.ContextVar('cvar')
+
+        def sub(num):
+            for i in range(10):
+                cvar.set(num + i)
+                time.sleep(random.uniform(0.001, 0.05))
+                self.assertEqual(cvar.get(), num + i)
+            return num
+
+        tp = concurrent.futures.ThreadPoolExecutor(max_workers=10)
+        try:
+            results = list(tp.map(sub, range(10)))
+        finally:
+            tp.shutdown()
+        self.assertEqual(results, list(range(10)))
+
+
+# HAMT Tests
+
+
+class HashKey:
+    _crasher = None
+
+    def __init__(self, hash, name, *, error_on_eq_to=None):
+        assert hash != -1
+        self.name = name
+        self.hash = hash
+        self.error_on_eq_to = error_on_eq_to
+
+    def __repr__(self):
+        return f'<Key name:{self.name} hash:{self.hash}>'
+
+    def __hash__(self):
+        if self._crasher is not None and self._crasher.error_on_hash:
+            raise HashingError
+
+        return self.hash
+
+    def __eq__(self, other):
+        if not isinstance(other, HashKey):
+            return NotImplemented
+
+        if self._crasher is not None and self._crasher.error_on_eq:
+            raise EqError
+
+        if self.error_on_eq_to is not None and self.error_on_eq_to is other:
+            raise ValueError(f'cannot compare {self!r} to {other!r}')
+        if other.error_on_eq_to is not None and other.error_on_eq_to is self:
+            raise ValueError(f'cannot compare {other!r} to {self!r}')
+
+        return (self.name, self.hash) == (other.name, other.hash)
+
+
+class KeyStr(str):
+    def __hash__(self):
+        if HashKey._crasher is not None and HashKey._crasher.error_on_hash:
+            raise HashingError
+        return super().__hash__()
+
+    def __eq__(self, other):
+        if HashKey._crasher is not None and HashKey._crasher.error_on_eq:
+            raise EqError
+        return super().__eq__(other)
+
+
+class HaskKeyCrasher:
+    def __init__(self, *, error_on_hash=False, error_on_eq=False):
+        self.error_on_hash = error_on_hash
+        self.error_on_eq = error_on_eq
+
+    def __enter__(self):
+        if HashKey._crasher is not None:
+            raise RuntimeError('cannot nest crashers')
+        HashKey._crasher = self
+
+    def __exit__(self, *exc):
+        HashKey._crasher = None
+
+
+class HashingError(Exception):
+    pass
+
+
+class EqError(Exception):
+    pass
+
+
+@unittest.skipIf(hamt is None, '_testcapi lacks "hamt()" function')
+class HamtTest(unittest.TestCase):
+
+    def test_hashkey_helper_1(self):
+        k1 = HashKey(10, 'aaa')
+        k2 = HashKey(10, 'bbb')
+
+        self.assertNotEqual(k1, k2)
+        self.assertEqual(hash(k1), hash(k2))
+
+        d = dict()
+        d[k1] = 'a'
+        d[k2] = 'b'
+
+        self.assertEqual(d[k1], 'a')
+        self.assertEqual(d[k2], 'b')
+
+    def test_hamt_basics_1(self):
+        h = hamt()
+        h = None  # NoQA
+
+    def test_hamt_basics_2(self):
+        h = hamt()
+        self.assertEqual(len(h), 0)
+
+        h2 = h.set('a', 'b')
+        self.assertIsNot(h, h2)
+        self.assertEqual(len(h), 0)
+        self.assertEqual(len(h2), 1)
+
+        self.assertIsNone(h.get('a'))
+        self.assertEqual(h.get('a', 42), 42)
+
+        self.assertEqual(h2.get('a'), 'b')
+
+        h3 = h2.set('b', 10)
+        self.assertIsNot(h2, h3)
+        self.assertEqual(len(h), 0)
+        self.assertEqual(len(h2), 1)
+        self.assertEqual(len(h3), 2)
+        self.assertEqual(h3.get('a'), 'b')
+        self.assertEqual(h3.get('b'), 10)
+
+        self.assertIsNone(h.get('b'))
+        self.assertIsNone(h2.get('b'))
+
+        self.assertIsNone(h.get('a'))
+        self.assertEqual(h2.get('a'), 'b')
+
+        h = h2 = h3 = None
+
+    def test_hamt_basics_3(self):
+        h = hamt()
+        o = object()
+        h1 = h.set('1', o)
+        h2 = h1.set('1', o)
+        self.assertIs(h1, h2)
+
+    def test_hamt_basics_4(self):
+        h = hamt()
+        h1 = h.set('key', [])
+        h2 = h1.set('key', [])
+        self.assertIsNot(h1, h2)
+        self.assertEqual(len(h1), 1)
+        self.assertEqual(len(h2), 1)
+        self.assertIsNot(h1.get('key'), h2.get('key'))
+
+    def test_hamt_collision_1(self):
+        k1 = HashKey(10, 'aaa')
+        k2 = HashKey(10, 'bbb')
+        k3 = HashKey(10, 'ccc')
+
+        h = hamt()
+        h2 = h.set(k1, 'a')
+        h3 = h2.set(k2, 'b')
+
+        self.assertEqual(h.get(k1), None)
+        self.assertEqual(h.get(k2), None)
+
+        self.assertEqual(h2.get(k1), 'a')
+        self.assertEqual(h2.get(k2), None)
+
+        self.assertEqual(h3.get(k1), 'a')
+        self.assertEqual(h3.get(k2), 'b')
+
+        h4 = h3.set(k2, 'cc')
+        h5 = h4.set(k3, 'aa')
+
+        self.assertEqual(h3.get(k1), 'a')
+        self.assertEqual(h3.get(k2), 'b')
+        self.assertEqual(h4.get(k1), 'a')
+        self.assertEqual(h4.get(k2), 'cc')
+        self.assertEqual(h4.get(k3), None)
+        self.assertEqual(h5.get(k1), 'a')
+        self.assertEqual(h5.get(k2), 'cc')
+        self.assertEqual(h5.get(k2), 'cc')
+        self.assertEqual(h5.get(k3), 'aa')
+
+        self.assertEqual(len(h), 0)
+        self.assertEqual(len(h2), 1)
+        self.assertEqual(len(h3), 2)
+        self.assertEqual(len(h4), 2)
+        self.assertEqual(len(h5), 3)
+
+    def test_hamt_stress(self):
+        COLLECTION_SIZE = 7000
+        TEST_ITERS_EVERY = 647
+        CRASH_HASH_EVERY = 97
+        CRASH_EQ_EVERY = 11
+        RUN_XTIMES = 3
+
+        for _ in range(RUN_XTIMES):
+            h = hamt()
+            d = dict()
+
+            for i in range(COLLECTION_SIZE):
+                key = KeyStr(i)
+
+                if not (i % CRASH_HASH_EVERY):
+                    with HaskKeyCrasher(error_on_hash=True):
+                        with self.assertRaises(HashingError):
+                            h.set(key, i)
+
+                h = h.set(key, i)
+
+                if not (i % CRASH_EQ_EVERY):
+                    with HaskKeyCrasher(error_on_eq=True):
+                        with self.assertRaises(EqError):
+                            h.get(KeyStr(i))  # really trigger __eq__
+
+                d[key] = i
+                self.assertEqual(len(d), len(h))
+
+                if not (i % TEST_ITERS_EVERY):
+                    self.assertEqual(set(h.items()), set(d.items()))
+                    self.assertEqual(len(h.items()), len(d.items()))
+
+            self.assertEqual(len(h), COLLECTION_SIZE)
+
+            for key in range(COLLECTION_SIZE):
+                self.assertEqual(h.get(KeyStr(key), 'not found'), key)
+
+            keys_to_delete = list(range(COLLECTION_SIZE))
+            random.shuffle(keys_to_delete)
+            for iter_i, i in enumerate(keys_to_delete):
+                key = KeyStr(i)
+
+                if not (iter_i % CRASH_HASH_EVERY):
+                    with HaskKeyCrasher(error_on_hash=True):
+                        with self.assertRaises(HashingError):
+                            h.delete(key)
+
+                if not (iter_i % CRASH_EQ_EVERY):
+                    with HaskKeyCrasher(error_on_eq=True):
+                        with self.assertRaises(EqError):
+                            h.delete(KeyStr(i))
+
+                h = h.delete(key)
+                self.assertEqual(h.get(key, 'not found'), 'not found')
+                del d[key]
+                self.assertEqual(len(d), len(h))
+
+                if iter_i == COLLECTION_SIZE // 2:
+                    hm = h
+                    dm = d.copy()
+
+                if not (iter_i % TEST_ITERS_EVERY):
+                    self.assertEqual(set(h.keys()), set(d.keys()))
+                    self.assertEqual(len(h.keys()), len(d.keys()))
+
+            self.assertEqual(len(d), 0)
+            self.assertEqual(len(h), 0)
+
+            # ============
+
+            for key in dm:
+                self.assertEqual(hm.get(str(key)), dm[key])
+            self.assertEqual(len(dm), len(hm))
+
+            for i, key in enumerate(keys_to_delete):
+                hm = hm.delete(str(key))
+                self.assertEqual(hm.get(str(key), 'not found'), 'not found')
+                dm.pop(str(key), None)
+                self.assertEqual(len(d), len(h))
+
+                if not (i % TEST_ITERS_EVERY):
+                    self.assertEqual(set(h.values()), set(d.values()))
+                    self.assertEqual(len(h.values()), len(d.values()))
+
+            self.assertEqual(len(d), 0)
+            self.assertEqual(len(h), 0)
+            self.assertEqual(list(h.items()), [])
+
+    def test_hamt_delete_1(self):
+        A = HashKey(100, 'A')
+        B = HashKey(101, 'B')
+        C = HashKey(102, 'C')
+        D = HashKey(103, 'D')
+        E = HashKey(104, 'E')
+        Z = HashKey(-100, 'Z')
+
+        Er = HashKey(103, 'Er', error_on_eq_to=D)
+
+        h = hamt()
+        h = h.set(A, 'a')
+        h = h.set(B, 'b')
+        h = h.set(C, 'c')
+        h = h.set(D, 'd')
+        h = h.set(E, 'e')
+
+        orig_len = len(h)
+
+        # BitmapNode(size=10 bitmap=0b111110000 id=0x10eadc618):
+        #     <Key name:A hash:100>: 'a'
+        #     <Key name:B hash:101>: 'b'
+        #     <Key name:C hash:102>: 'c'
+        #     <Key name:D hash:103>: 'd'
+        #     <Key name:E hash:104>: 'e'
+
+        h = h.delete(C)
+        self.assertEqual(len(h), orig_len - 1)
+
+        with self.assertRaisesRegex(ValueError, 'cannot compare'):
+            h.delete(Er)
+
+        h = h.delete(D)
+        self.assertEqual(len(h), orig_len - 2)
+
+        h2 = h.delete(Z)
+        self.assertIs(h2, h)
+
+        h = h.delete(A)
+        self.assertEqual(len(h), orig_len - 3)
+
+        self.assertEqual(h.get(A, 42), 42)
+        self.assertEqual(h.get(B), 'b')
+        self.assertEqual(h.get(E), 'e')
+
+    def test_hamt_delete_2(self):
+        A = HashKey(100, 'A')
+        B = HashKey(201001, 'B')
+        C = HashKey(101001, 'C')
+        D = HashKey(103, 'D')
+        E = HashKey(104, 'E')
+        Z = HashKey(-100, 'Z')
+
+        Er = HashKey(201001, 'Er', error_on_eq_to=B)
+
+        h = hamt()
+        h = h.set(A, 'a')
+        h = h.set(B, 'b')
+        h = h.set(C, 'c')
+        h = h.set(D, 'd')
+        h = h.set(E, 'e')
+
+        orig_len = len(h)
+
+        # BitmapNode(size=8 bitmap=0b1110010000):
+        #     <Key name:A hash:100>: 'a'
+        #     <Key name:D hash:103>: 'd'
+        #     <Key name:E hash:104>: 'e'
+        #     NULL:
+        #         BitmapNode(size=4 bitmap=0b100000000001000000000):
+        #             <Key name:B hash:201001>: 'b'
+        #             <Key name:C hash:101001>: 'c'
+
+        with self.assertRaisesRegex(ValueError, 'cannot compare'):
+            h.delete(Er)
+
+        h = h.delete(Z)
+        self.assertEqual(len(h), orig_len)
+
+        h = h.delete(C)
+        self.assertEqual(len(h), orig_len - 1)
+
+        h = h.delete(B)
+        self.assertEqual(len(h), orig_len - 2)
+
+        h = h.delete(A)
+        self.assertEqual(len(h), orig_len - 3)
+
+        self.assertEqual(h.get(D), 'd')
+        self.assertEqual(h.get(E), 'e')
+
+        h = h.delete(A)
+        h = h.delete(B)
+        h = h.delete(D)
+        h = h.delete(E)
+        self.assertEqual(len(h), 0)
+
+    def test_hamt_delete_3(self):
+        A = HashKey(100, 'A')
+        B = HashKey(101, 'B')
+        C = HashKey(100100, 'C')
+        D = HashKey(100100, 'D')
+        E = HashKey(104, 'E')
+
+        h = hamt()
+        h = h.set(A, 'a')
+        h = h.set(B, 'b')
+        h = h.set(C, 'c')
+        h = h.set(D, 'd')
+        h = h.set(E, 'e')
+
+        orig_len = len(h)
+
+        # BitmapNode(size=6 bitmap=0b100110000):
+        #     NULL:
+        #         BitmapNode(size=4 bitmap=0b1000000000000000000001000):
+        #             <Key name:A hash:100>: 'a'
+        #             NULL:
+        #                 CollisionNode(size=4 id=0x108572410):
+        #                     <Key name:C hash:100100>: 'c'
+        #                     <Key name:D hash:100100>: 'd'
+        #     <Key name:B hash:101>: 'b'
+        #     <Key name:E hash:104>: 'e'
+
+        h = h.delete(A)
+        self.assertEqual(len(h), orig_len - 1)
+
+        h = h.delete(E)
+        self.assertEqual(len(h), orig_len - 2)
+
+        self.assertEqual(h.get(C), 'c')
+        self.assertEqual(h.get(B), 'b')
+
+    def test_hamt_delete_4(self):
+        A = HashKey(100, 'A')
+        B = HashKey(101, 'B')
+        C = HashKey(100100, 'C')
+        D = HashKey(100100, 'D')
+        E = HashKey(100100, 'E')
+
+        h = hamt()
+        h = h.set(A, 'a')
+        h = h.set(B, 'b')
+        h = h.set(C, 'c')
+        h = h.set(D, 'd')
+        h = h.set(E, 'e')
+
+        orig_len = len(h)
+
+        # BitmapNode(size=4 bitmap=0b110000):
+        #     NULL:
+        #         BitmapNode(size=4 bitmap=0b1000000000000000000001000):
+        #             <Key name:A hash:100>: 'a'
+        #             NULL:
+        #                 CollisionNode(size=6 id=0x10515ef30):
+        #                     <Key name:C hash:100100>: 'c'
+        #                     <Key name:D hash:100100>: 'd'
+        #                     <Key name:E hash:100100>: 'e'
+        #     <Key name:B hash:101>: 'b'
+
+        h = h.delete(D)
+        self.assertEqual(len(h), orig_len - 1)
+
+        h = h.delete(E)
+        self.assertEqual(len(h), orig_len - 2)
+
+        h = h.delete(C)
+        self.assertEqual(len(h), orig_len - 3)
+
+        h = h.delete(A)
+        self.assertEqual(len(h), orig_len - 4)
+
+        h = h.delete(B)
+        self.assertEqual(len(h), 0)
+
+    def test_hamt_delete_5(self):
+        h = hamt()
+
+        keys = []
+        for i in range(17):
+            key = HashKey(i, str(i))
+            keys.append(key)
+            h = h.set(key, f'val-{i}')
+
+        collision_key16 = HashKey(16, '18')
+        h = h.set(collision_key16, 'collision')
+
+        # ArrayNode(id=0x10f8b9318):
+        #     0::
+        #     BitmapNode(size=2 count=1 bitmap=0b1):
+        #         <Key name:0 hash:0>: 'val-0'
+        #
+        # ... 14 more BitmapNodes ...
+        #
+        #     15::
+        #     BitmapNode(size=2 count=1 bitmap=0b1):
+        #         <Key name:15 hash:15>: 'val-15'
+        #
+        #     16::
+        #     BitmapNode(size=2 count=1 bitmap=0b1):
+        #         NULL:
+        #             CollisionNode(size=4 id=0x10f2f5af8):
+        #                 <Key name:16 hash:16>: 'val-16'
+        #                 <Key name:18 hash:16>: 'collision'
+
+        self.assertEqual(len(h), 18)
+
+        h = h.delete(keys[2])
+        self.assertEqual(len(h), 17)
+
+        h = h.delete(collision_key16)
+        self.assertEqual(len(h), 16)
+        h = h.delete(keys[16])
+        self.assertEqual(len(h), 15)
+
+        h = h.delete(keys[1])
+        self.assertEqual(len(h), 14)
+        h = h.delete(keys[1])
+        self.assertEqual(len(h), 14)
+
+        for key in keys:
+            h = h.delete(key)
+        self.assertEqual(len(h), 0)
+
+    def test_hamt_items_1(self):
+        A = HashKey(100, 'A')
+        B = HashKey(201001, 'B')
+        C = HashKey(101001, 'C')
+        D = HashKey(103, 'D')
+        E = HashKey(104, 'E')
+        F = HashKey(110, 'F')
+
+        h = hamt()
+        h = h.set(A, 'a')
+        h = h.set(B, 'b')
+        h = h.set(C, 'c')
+        h = h.set(D, 'd')
+        h = h.set(E, 'e')
+        h = h.set(F, 'f')
+
+        it = h.items()
+        self.assertEqual(
+            set(list(it)),
+            {(A, 'a'), (B, 'b'), (C, 'c'), (D, 'd'), (E, 'e'), (F, 'f')})
+
+    def test_hamt_items_2(self):
+        A = HashKey(100, 'A')
+        B = HashKey(101, 'B')
+        C = HashKey(100100, 'C')
+        D = HashKey(100100, 'D')
+        E = HashKey(100100, 'E')
+        F = HashKey(110, 'F')
+
+        h = hamt()
+        h = h.set(A, 'a')
+        h = h.set(B, 'b')
+        h = h.set(C, 'c')
+        h = h.set(D, 'd')
+        h = h.set(E, 'e')
+        h = h.set(F, 'f')
+
+        it = h.items()
+        self.assertEqual(
+            set(list(it)),
+            {(A, 'a'), (B, 'b'), (C, 'c'), (D, 'd'), (E, 'e'), (F, 'f')})
+
+    def test_hamt_keys_1(self):
+        A = HashKey(100, 'A')
+        B = HashKey(101, 'B')
+        C = HashKey(100100, 'C')
+        D = HashKey(100100, 'D')
+        E = HashKey(100100, 'E')
+        F = HashKey(110, 'F')
+
+        h = hamt()
+        h = h.set(A, 'a')
+        h = h.set(B, 'b')
+        h = h.set(C, 'c')
+        h = h.set(D, 'd')
+        h = h.set(E, 'e')
+        h = h.set(F, 'f')
+
+        self.assertEqual(set(list(h.keys())), {A, B, C, D, E, F})
+        self.assertEqual(set(list(h)), {A, B, C, D, E, F})
+
+    def test_hamt_items_3(self):
+        h = hamt()
+        self.assertEqual(len(h.items()), 0)
+        self.assertEqual(list(h.items()), [])
+
+    def test_hamt_eq_1(self):
+        A = HashKey(100, 'A')
+        B = HashKey(101, 'B')
+        C = HashKey(100100, 'C')
+        D = HashKey(100100, 'D')
+        E = HashKey(120, 'E')
+
+        h1 = hamt()
+        h1 = h1.set(A, 'a')
+        h1 = h1.set(B, 'b')
+        h1 = h1.set(C, 'c')
+        h1 = h1.set(D, 'd')
+
+        h2 = hamt()
+        h2 = h2.set(A, 'a')
+
+        self.assertFalse(h1 == h2)
+        self.assertTrue(h1 != h2)
+
+        h2 = h2.set(B, 'b')
+        self.assertFalse(h1 == h2)
+        self.assertTrue(h1 != h2)
+
+        h2 = h2.set(C, 'c')
+        self.assertFalse(h1 == h2)
+        self.assertTrue(h1 != h2)
+
+        h2 = h2.set(D, 'd2')
+        self.assertFalse(h1 == h2)
+        self.assertTrue(h1 != h2)
+
+        h2 = h2.set(D, 'd')
+        self.assertTrue(h1 == h2)
+        self.assertFalse(h1 != h2)
+
+        h2 = h2.set(E, 'e')
+        self.assertFalse(h1 == h2)
+        self.assertTrue(h1 != h2)
+
+        h2 = h2.delete(D)
+        self.assertFalse(h1 == h2)
+        self.assertTrue(h1 != h2)
+
+        h2 = h2.set(E, 'd')
+        self.assertFalse(h1 == h2)
+        self.assertTrue(h1 != h2)
+
+    def test_hamt_eq_2(self):
+        A = HashKey(100, 'A')
+        Er = HashKey(100, 'Er', error_on_eq_to=A)
+
+        h1 = hamt()
+        h1 = h1.set(A, 'a')
+
+        h2 = hamt()
+        h2 = h2.set(Er, 'a')
+
+        with self.assertRaisesRegex(ValueError, 'cannot compare'):
+            h1 == h2
+
+        with self.assertRaisesRegex(ValueError, 'cannot compare'):
+            h1 != h2
+
+    def test_hamt_gc_1(self):
+        A = HashKey(100, 'A')
+
+        h = hamt()
+        h = h.set(0, 0)  # empty HAMT node is memoized in hamt.c
+        ref = weakref.ref(h)
+
+        a = []
+        a.append(a)
+        a.append(h)
+        b = []
+        a.append(b)
+        b.append(a)
+        h = h.set(A, b)
+
+        del h, a, b
+
+        gc.collect()
+        gc.collect()
+        gc.collect()
+
+        self.assertIsNone(ref())
+
+    def test_hamt_gc_2(self):
+        A = HashKey(100, 'A')
+        B = HashKey(101, 'B')
+
+        h = hamt()
+        h = h.set(A, 'a')
+        h = h.set(A, h)
+
+        ref = weakref.ref(h)
+        hi = h.items()
+        next(hi)
+
+        del h, hi
+
+        gc.collect()
+        gc.collect()
+        gc.collect()
+
+        self.assertIsNone(ref())
+
+    def test_hamt_in_1(self):
+        A = HashKey(100, 'A')
+        AA = HashKey(100, 'A')
+
+        B = HashKey(101, 'B')
+
+        h = hamt()
+        h = h.set(A, 1)
+
+        self.assertTrue(A in h)
+        self.assertFalse(B in h)
+
+        with self.assertRaises(EqError):
+            with HaskKeyCrasher(error_on_eq=True):
+                AA in h
+
+        with self.assertRaises(HashingError):
+            with HaskKeyCrasher(error_on_hash=True):
+                AA in h
+
+    def test_hamt_getitem_1(self):
+        A = HashKey(100, 'A')
+        AA = HashKey(100, 'A')
+
+        B = HashKey(101, 'B')
+
+        h = hamt()
+        h = h.set(A, 1)
+
+        self.assertEqual(h[A], 1)
+        self.assertEqual(h[AA], 1)
+
+        with self.assertRaises(KeyError):
+            h[B]
+
+        with self.assertRaises(EqError):
+            with HaskKeyCrasher(error_on_eq=True):
+                h[AA]
+
+        with self.assertRaises(HashingError):
+            with HaskKeyCrasher(error_on_hash=True):
+                h[AA]
+
+
+if __name__ == "__main__":
+    unittest.main()
diff --git a/Makefile.pre.in b/Makefile.pre.in
index d151267173..162006a2f9 100644
--- a/Makefile.pre.in
+++ b/Makefile.pre.in
@@ -354,6 +354,8 @@ PYTHON_OBJS=	\
 		Python/pylifecycle.o \
 		Python/pymath.o \
 		Python/pystate.o \
+		Python/context.o \
+		Python/hamt.o \
 		Python/pythonrun.o \
 		Python/pytime.o \
 		Python/bootstrap_hash.o \
@@ -996,6 +998,7 @@ PYTHON_HEADERS= \
 		$(srcdir)/Include/pymem.h \
 		$(srcdir)/Include/pyport.h \
 		$(srcdir)/Include/pystate.h \
+		$(srcdir)/Include/context.h \
 		$(srcdir)/Include/pystrcmp.h \
 		$(srcdir)/Include/pystrtod.h \
 		$(srcdir)/Include/pystrhex.h \
@@ -1023,6 +1026,7 @@ PYTHON_HEADERS= \
 		$(srcdir)/Include/internal/mem.h \
 		$(srcdir)/Include/internal/pygetopt.h \
 		$(srcdir)/Include/internal/pystate.h \
+		$(srcdir)/Include/internal/context.h \
 		$(srcdir)/Include/internal/warnings.h \
 		$(DTRACE_HEADERS)
 
diff --git a/Misc/NEWS.d/next/Core and Builtins/2017-12-28-00-20-42.bpo-32436.H159Jv.rst b/Misc/NEWS.d/next/Core and Builtins/2017-12-28-00-20-42.bpo-32436.H159Jv.rst
new file mode 100644
index 0000000000..8586d77178
--- /dev/null
+++ b/Misc/NEWS.d/next/Core and Builtins/2017-12-28-00-20-42.bpo-32436.H159Jv.rst
@@ -0,0 +1 @@
+Implement PEP 567
diff --git a/Modules/Setup.dist b/Modules/Setup.dist
index 1f2d56c065..239550e26b 100644
--- a/Modules/Setup.dist
+++ b/Modules/Setup.dist
@@ -176,6 +176,7 @@ _symtable symtablemodule.c
 #array arraymodule.c	# array objects
 #cmath cmathmodule.c _math.c # -lm # complex math library functions
 #math mathmodule.c _math.c # -lm # math library functions, e.g. sin()
+#_contextvars _contextvarsmodule.c  # Context Variables
 #_struct _struct.c	# binary structure packing/unpacking
 #_weakref _weakref.c	# basic weak reference support
 #_testcapi _testcapimodule.c    # Python C API test module
diff --git a/Modules/_asynciomodule.c b/Modules/_asynciomodule.c
index 22ce32c593..f77ec999b0 100644
--- a/Modules/_asynciomodule.c
+++ b/Modules/_asynciomodule.c
@@ -36,6 +36,7 @@ static PyObject *asyncio_task_print_stack_func;
 static PyObject *asyncio_task_repr_info_func;
 static PyObject *asyncio_InvalidStateError;
 static PyObject *asyncio_CancelledError;
+static PyObject *context_kwname;
 
 
 /* WeakSet containing all alive tasks. */
@@ -59,6 +60,7 @@ typedef enum {
     PyObject_HEAD                                                           \
     PyObject *prefix##_loop;                                                \
     PyObject *prefix##_callback0;                                           \
+    PyContext *prefix##_context0;                                           \
     PyObject *prefix##_callbacks;                                           \
     PyObject *prefix##_exception;                                           \
     PyObject *prefix##_result;                                              \
@@ -77,6 +79,7 @@ typedef struct {
     FutureObj_HEAD(task)
     PyObject *task_fut_waiter;
     PyObject *task_coro;
+    PyContext *task_context;
     int task_must_cancel;
     int task_log_destroy_pending;
 } TaskObj;
@@ -336,11 +339,38 @@ get_event_loop(void)
 
 
 static int
-call_soon(PyObject *loop, PyObject *func, PyObject *arg)
+call_soon(PyObject *loop, PyObject *func, PyObject *arg, PyContext *ctx)
 {
     PyObject *handle;
-    handle = _PyObject_CallMethodIdObjArgs(
-        loop, &PyId_call_soon, func, arg, NULL);
+    PyObject *stack[3];
+    Py_ssize_t nargs;
+
+    if (ctx == NULL) {
+        handle = _PyObject_CallMethodIdObjArgs(
+            loop, &PyId_call_soon, func, arg, NULL);
+    }
+    else {
+        /* Use FASTCALL to pass a keyword-only argument to call_soon */
+
+        PyObject *callable = _PyObject_GetAttrId(loop, &PyId_call_soon);
+        if (callable == NULL) {
+            return -1;
+        }
+
+        /* All refs in 'stack' are borrowed. */
+        nargs = 1;
+        stack[0] = func;
+        if (arg != NULL) {
+            stack[1] = arg;
+            nargs++;
+        }
+        stack[nargs] = (PyObject *)ctx;
+
+        handle = _PyObject_FastCallKeywords(
+            callable, stack, nargs, context_kwname);
+        Py_DECREF(callable);
+    }
+
     if (handle == NULL) {
         return -1;
     }
@@ -387,8 +417,11 @@ future_schedule_callbacks(FutureObj *fut)
         /* There's a 1st callback */
 
         int ret = call_soon(
-            fut->fut_loop, fut->fut_callback0, (PyObject *)fut);
+            fut->fut_loop, fut->fut_callback0,
+            (PyObject *)fut, fut->fut_context0);
+
         Py_CLEAR(fut->fut_callback0);
+        Py_CLEAR(fut->fut_context0);
         if (ret) {
             /* If an error occurs in pure-Python implementation,
                all callbacks are cleared. */
@@ -413,9 +446,11 @@ future_schedule_callbacks(FutureObj *fut)
     }
 
     for (i = 0; i < len; i++) {
-        PyObject *cb = PyList_GET_ITEM(fut->fut_callbacks, i);
+        PyObject *cb_tup = PyList_GET_ITEM(fut->fut_callbacks, i);
+        PyObject *cb = PyTuple_GET_ITEM(cb_tup, 0);
+        PyObject *ctx = PyTuple_GET_ITEM(cb_tup, 1);
 
-        if (call_soon(fut->fut_loop, cb, (PyObject *)fut)) {
+        if (call_soon(fut->fut_loop, cb, (PyObject *)fut, (PyContext *)ctx)) {
             /* If an error occurs in pure-Python implementation,
                all callbacks are cleared. */
             Py_CLEAR(fut->fut_callbacks);
@@ -462,6 +497,7 @@ future_init(FutureObj *fut, PyObject *loop)
     }
 
     fut->fut_callback0 = NULL;
+    fut->fut_context0 = NULL;
     fut->fut_callbacks = NULL;
 
     return 0;
@@ -566,7 +602,7 @@ future_get_result(FutureObj *fut, PyObject **result)
 }
 
 static PyObject *
-future_add_done_callback(FutureObj *fut, PyObject *arg)
+future_add_done_callback(FutureObj *fut, PyObject *arg, PyContext *ctx)
 {
     if (!future_is_alive(fut)) {
         PyErr_SetString(PyExc_RuntimeError, "uninitialized Future object");
@@ -576,7 +612,7 @@ future_add_done_callback(FutureObj *fut, PyObject *arg)
     if (fut->fut_state != STATE_PENDING) {
         /* The future is done/cancelled, so schedule the callback
            right away. */
-        if (call_soon(fut->fut_loop, arg, (PyObject*) fut)) {
+        if (call_soon(fut->fut_loop, arg, (PyObject*) fut, ctx)) {
             return NULL;
         }
     }
@@ -602,24 +638,38 @@ future_add_done_callback(FutureObj *fut, PyObject *arg)
                 with a new list and add the new callback to it.
         */
 
-        if (fut->fut_callbacks != NULL) {
-            int err = PyList_Append(fut->fut_callbacks, arg);
-            if (err != 0) {
-                return NULL;
-            }
-        }
-        else if (fut->fut_callback0 == NULL) {
+        if (fut->fut_callbacks == NULL && fut->fut_callback0 == NULL) {
             Py_INCREF(arg);
             fut->fut_callback0 = arg;
+            Py_INCREF(ctx);
+            fut->fut_context0 = ctx;
         }
         else {
-            fut->fut_callbacks = PyList_New(1);
-            if (fut->fut_callbacks == NULL) {
+            PyObject *tup = PyTuple_New(2);
+            if (tup == NULL) {
                 return NULL;
             }
-
             Py_INCREF(arg);
-            PyList_SET_ITEM(fut->fut_callbacks, 0, arg);
+            PyTuple_SET_ITEM(tup, 0, arg);
+            Py_INCREF(ctx);
+            PyTuple_SET_ITEM(tup, 1, (PyObject *)ctx);
+
+            if (fut->fut_callbacks != NULL) {
+                int err = PyList_Append(fut->fut_callbacks, tup);
+                if (err) {
+                    Py_DECREF(tup);
+                    return NULL;
+                }
+                Py_DECREF(tup);
+            }
+            else {
+                fut->fut_callbacks = PyList_New(1);
+                if (fut->fut_callbacks == NULL) {
+                    return NULL;
+                }
+
+                PyList_SET_ITEM(fut->fut_callbacks, 0, tup);  /* borrow */
+            }
         }
     }
 
@@ -676,6 +726,7 @@ FutureObj_clear(FutureObj *fut)
 {
     Py_CLEAR(fut->fut_loop);
     Py_CLEAR(fut->fut_callback0);
+    Py_CLEAR(fut->fut_context0);
     Py_CLEAR(fut->fut_callbacks);
     Py_CLEAR(fut->fut_result);
     Py_CLEAR(fut->fut_exception);
@@ -689,6 +740,7 @@ FutureObj_traverse(FutureObj *fut, visitproc visit, void *arg)
 {
     Py_VISIT(fut->fut_loop);
     Py_VISIT(fut->fut_callback0);
+    Py_VISIT(fut->fut_context0);
     Py_VISIT(fut->fut_callbacks);
     Py_VISIT(fut->fut_result);
     Py_VISIT(fut->fut_exception);
@@ -821,6 +873,8 @@ _asyncio.Future.add_done_callback
 
     fn: object
     /
+    *
+    context: object = NULL
 
 Add a callback to be run when the future becomes done.
 
@@ -830,10 +884,21 @@ scheduled with call_soon.
 [clinic start generated code]*/
 
 static PyObject *
-_asyncio_Future_add_done_callback(FutureObj *self, PyObject *fn)
-/*[clinic end generated code: output=819e09629b2ec2b5 input=8f818b39990b027d]*/
+_asyncio_Future_add_done_callback_impl(FutureObj *self, PyObject *fn,
+                                       PyObject *context)
+/*[clinic end generated code: output=7ce635bbc9554c1e input=15ab0693a96e9533]*/
 {
-    return future_add_done_callback(self, fn);
+    if (context == NULL) {
+        context = (PyObject *)PyContext_CopyCurrent();
+        if (context == NULL) {
+            return NULL;
+        }
+        PyObject *res = future_add_done_callback(
+            self, fn, (PyContext *)context);
+        Py_DECREF(context);
+        return res;
+    }
+    return future_add_done_callback(self, fn, (PyContext *)context);
 }
 
 /*[clinic input]
@@ -865,6 +930,7 @@ _asyncio_Future_remove_done_callback(FutureObj *self, PyObject *fn)
         if (cmp == 1) {
             /* callback0 == fn */
             Py_CLEAR(self->fut_callback0);
+            Py_CLEAR(self->fut_context0);
             cleared_callback0 = 1;
         }
     }
@@ -880,8 +946,9 @@ _asyncio_Future_remove_done_callback(FutureObj *self, PyObject *fn)
     }
 
     if (len == 1) {
+        PyObject *cb_tup = PyList_GET_ITEM(self->fut_callbacks, 0);
         int cmp = PyObject_RichCompareBool(
-            fn, PyList_GET_ITEM(self->fut_callbacks, 0), Py_EQ);
+            fn, PyTuple_GET_ITEM(cb_tup, 0), Py_EQ);
         if (cmp == -1) {
             return NULL;
         }
@@ -903,7 +970,7 @@ _asyncio_Future_remove_done_callback(FutureObj *self, PyObject *fn)
         int ret;
         PyObject *item = PyList_GET_ITEM(self->fut_callbacks, i);
         Py_INCREF(item);
-        ret = PyObject_RichCompareBool(fn, item, Py_EQ);
+        ret = PyObject_RichCompareBool(fn, PyTuple_GET_ITEM(item, 0), Py_EQ);
         if (ret == 0) {
             if (j < len) {
                 PyList_SET_ITEM(newlist, j, item);
@@ -1081,47 +1148,49 @@ static PyObject *
 FutureObj_get_callbacks(FutureObj *fut)
 {
     Py_ssize_t i;
-    Py_ssize_t len;
-    PyObject *new_list;
 
     ENSURE_FUTURE_ALIVE(fut)
 
-    if (fut->fut_callbacks == NULL) {
-        if (fut->fut_callback0 == NULL) {
+    if (fut->fut_callback0 == NULL) {
+        if (fut->fut_callbacks == NULL) {
             Py_RETURN_NONE;
         }
-        else {
-            new_list = PyList_New(1);
-            if (new_list == NULL) {
-                return NULL;
-            }
-            Py_INCREF(fut->fut_callback0);
-            PyList_SET_ITEM(new_list, 0, fut->fut_callback0);
-            return new_list;
-        }
-    }
 
-    assert(fut->fut_callbacks != NULL);
-
-    if (fut->fut_callback0 == NULL) {
         Py_INCREF(fut->fut_callbacks);
         return fut->fut_callbacks;
     }
 
-    assert(fut->fut_callback0 != NULL);
+    Py_ssize_t len = 1;
+    if (fut->fut_callbacks != NULL) {
+        len += PyList_GET_SIZE(fut->fut_callbacks);
+    }
 
-    len = PyList_GET_SIZE(fut->fut_callbacks);
-    new_list = PyList_New(len + 1);
+
+    PyObject *new_list = PyList_New(len);
     if (new_list == NULL) {
         return NULL;
     }
 
+    PyObject *tup0 = PyTuple_New(2);
+    if (tup0 == NULL) {
+        Py_DECREF(new_list);
+        return NULL;
+    }
+
     Py_INCREF(fut->fut_callback0);
-    PyList_SET_ITEM(new_list, 0, fut->fut_callback0);
-    for (i = 0; i < len; i++) {
-        PyObject *cb = PyList_GET_ITEM(fut->fut_callbacks, i);
-        Py_INCREF(cb);
-        PyList_SET_ITEM(new_list, i + 1, cb);
+    PyTuple_SET_ITEM(tup0, 0, fut->fut_callback0);
+    assert(fut->fut_context0 != NULL);
+    Py_INCREF(fut->fut_context0);
+    PyTuple_SET_ITEM(tup0, 1, (PyObject *)fut->fut_context0);
+
+    PyList_SET_ITEM(new_list, 0, tup0);
+
+    if (fut->fut_callbacks != NULL) {
+        for (i = 0; i < PyList_GET_SIZE(fut->fut_callbacks); i++) {
+            PyObject *cb = PyList_GET_ITEM(fut->fut_callbacks, i);
+            Py_INCREF(cb);
+            PyList_SET_ITEM(new_list, i + 1, cb);
+        }
     }
 
     return new_list;
@@ -1912,6 +1981,11 @@ _asyncio_Task___init___impl(TaskObj *self, PyObject *coro, PyObject *loop)
         return -1;
     }
 
+    self->task_context = PyContext_CopyCurrent();
+    if (self->task_context == NULL) {
+        return -1;
+    }
+
     self->task_fut_waiter = NULL;
     self->task_must_cancel = 0;
     self->task_log_destroy_pending = 1;
@@ -1928,6 +2002,7 @@ static int
 TaskObj_clear(TaskObj *task)
 {
     (void)FutureObj_clear((FutureObj*) task);
+    Py_CLEAR(task->task_context);
     Py_CLEAR(task->task_coro);
     Py_CLEAR(task->task_fut_waiter);
     return 0;
@@ -1936,6 +2011,7 @@ TaskObj_clear(TaskObj *task)
 static int
 TaskObj_traverse(TaskObj *task, visitproc visit, void *arg)
 {
+    Py_VISIT(task->task_context);
     Py_VISIT(task->task_coro);
     Py_VISIT(task->task_fut_waiter);
     (void)FutureObj_traverse((FutureObj*) task, visit, arg);
@@ -2451,7 +2527,7 @@ task_call_step_soon(TaskObj *task, PyObject *arg)
         return -1;
     }
 
-    int ret = call_soon(task->task_loop, cb, NULL);
+    int ret = call_soon(task->task_loop, cb, NULL, task->task_context);
     Py_DECREF(cb);
     return ret;
 }
@@ -2650,7 +2726,8 @@ set_exception:
             if (wrapper == NULL) {
                 goto fail;
             }
-            res = future_add_done_callback((FutureObj*)result, wrapper);
+            res = future_add_done_callback(
+                (FutureObj*)result, wrapper, task->task_context);
             Py_DECREF(wrapper);
             if (res == NULL) {
                 goto fail;
@@ -2724,14 +2801,23 @@ set_exception:
                     goto fail;
                 }
 
-                /* result.add_done_callback(task._wakeup) */
                 wrapper = TaskWakeupMethWrapper_new(task);
                 if (wrapper == NULL) {
                     goto fail;
                 }
-                res = _PyObject_CallMethodIdObjArgs(result,
-                                                    &PyId_add_done_callback,
-                                                    wrapper, NULL);
+
+                /* result.add_done_callback(task._wakeup) */
+                PyObject *add_cb = _PyObject_GetAttrId(
+                    result, &PyId_add_done_callback);
+                if (add_cb == NULL) {
+                    goto fail;
+                }
+                PyObject *stack[2];
+                stack[0] = wrapper;
+                stack[1] = (PyObject *)task->task_context;
+                res = _PyObject_FastCallKeywords(
+                    add_cb, stack, 1, context_kwname);
+                Py_DECREF(add_cb);
                 Py_DECREF(wrapper);
                 if (res == NULL) {
                     goto fail;
@@ -3141,6 +3227,8 @@ module_free(void *m)
     Py_CLEAR(current_tasks);
     Py_CLEAR(iscoroutine_typecache);
 
+    Py_CLEAR(context_kwname);
+
     module_free_freelists();
 }
 
@@ -3164,6 +3252,17 @@ module_init(void)
         goto fail;
     }
 
+
+    context_kwname = PyTuple_New(1);
+    if (context_kwname == NULL) {
+        goto fail;
+    }
+    PyObject *context_str = PyUnicode_FromString("context");
+    if (context_str == NULL) {
+        goto fail;
+    }
+    PyTuple_SET_ITEM(context_kwname, 0, context_str);
+
 #define WITH_MOD(NAME) \
     Py_CLEAR(module); \
     module = PyImport_ImportModule(NAME); \
diff --git a/Modules/_contextvarsmodule.c b/Modules/_contextvarsmodule.c
new file mode 100644
index 0000000000..b7d112dd60
--- /dev/null
+++ b/Modules/_contextvarsmodule.c
@@ -0,0 +1,75 @@
+#include "Python.h"
+
+#include "clinic/_contextvarsmodule.c.h"
+
+/*[clinic input]
+module _contextvars
+[clinic start generated code]*/
+/*[clinic end generated code: output=da39a3ee5e6b4b0d input=a0955718c8b8cea6]*/
+
+
+/*[clinic input]
+_contextvars.copy_context
+[clinic start generated code]*/
+
+static PyObject *
+_contextvars_copy_context_impl(PyObject *module)
+/*[clinic end generated code: output=1fcd5da7225c4fa9 input=89bb9ae485888440]*/
+{
+    return (PyObject *)PyContext_CopyCurrent();
+}
+
+
+PyDoc_STRVAR(module_doc, "Context Variables");
+
+static PyMethodDef _contextvars_methods[] = {
+    _CONTEXTVARS_COPY_CONTEXT_METHODDEF
+    {NULL, NULL}
+};
+
+static struct PyModuleDef _contextvarsmodule = {
+    PyModuleDef_HEAD_INIT,      /* m_base */
+    "_contextvars",             /* m_name */
+    module_doc,                 /* m_doc */
+    -1,                         /* m_size */
+    _contextvars_methods,       /* m_methods */
+    NULL,                       /* m_slots */
+    NULL,                       /* m_traverse */
+    NULL,                       /* m_clear */
+    NULL,                       /* m_free */
+};
+
+PyMODINIT_FUNC
+PyInit__contextvars(void)
+{
+    PyObject *m = PyModule_Create(&_contextvarsmodule);
+    if (m == NULL) {
+        return NULL;
+    }
+
+    Py_INCREF(&PyContext_Type);
+    if (PyModule_AddObject(m, "Context",
+                           (PyObject *)&PyContext_Type) < 0)
+    {
+        Py_DECREF(&PyContext_Type);
+        return NULL;
+    }
+
+    Py_INCREF(&PyContextVar_Type);
+    if (PyModule_AddObject(m, "ContextVar",
+                           (PyObject *)&PyContextVar_Type) < 0)
+    {
+        Py_DECREF(&PyContextVar_Type);
+        return NULL;
+    }
+
+    Py_INCREF(&PyContextToken_Type);
+    if (PyModule_AddObject(m, "Token",
+                           (PyObject *)&PyContextToken_Type) < 0)
+    {
+        Py_DECREF(&PyContextToken_Type);
+        return NULL;
+    }
+
+    return m;
+}
diff --git a/Modules/_testcapimodule.c b/Modules/_testcapimodule.c
index 0d6bf45032..e3be7d3d82 100644
--- a/Modules/_testcapimodule.c
+++ b/Modules/_testcapimodule.c
@@ -4438,6 +4438,13 @@ test_pythread_tss_key_state(PyObject *self, PyObject *args)
 }
 
 
+static PyObject*
+new_hamt(PyObject *self, PyObject *args)
+{
+    return _PyContext_NewHamtForTests();
+}
+
+
 static PyMethodDef TestMethods[] = {
     {"raise_exception",         raise_exception,                 METH_VARARGS},
     {"raise_memoryerror",   (PyCFunction)raise_memoryerror,  METH_NOARGS},
@@ -4655,6 +4662,7 @@ static PyMethodDef TestMethods[] = {
     {"get_mapping_values", get_mapping_values, METH_O},
     {"get_mapping_items", get_mapping_items, METH_O},
     {"test_pythread_tss_key_state", test_pythread_tss_key_state, METH_VARARGS},
+    {"hamt", new_hamt, METH_NOARGS},
     {NULL, NULL} /* sentinel */
 };
 
diff --git a/Modules/clinic/_asynciomodule.c.h b/Modules/clinic/_asynciomodule.c.h
index f2e0f405ad..9fc9d6b17c 100644
--- a/Modules/clinic/_asynciomodule.c.h
+++ b/Modules/clinic/_asynciomodule.c.h
@@ -110,7 +110,7 @@ PyDoc_STRVAR(_asyncio_Future_set_exception__doc__,
     {"set_exception", (PyCFunction)_asyncio_Future_set_exception, METH_O, _asyncio_Future_set_exception__doc__},
 
 PyDoc_STRVAR(_asyncio_Future_add_done_callback__doc__,
-"add_done_callback($self, fn, /)\n"
+"add_done_callback($self, fn, /, *, context=None)\n"
 "--\n"
 "\n"
 "Add a callback to be run when the future becomes done.\n"
@@ -120,7 +120,30 @@ PyDoc_STRVAR(_asyncio_Future_add_done_callback__doc__,
 "scheduled with call_soon.");
 
 #define _ASYNCIO_FUTURE_ADD_DONE_CALLBACK_METHODDEF    \
-    {"add_done_callback", (PyCFunction)_asyncio_Future_add_done_callback, METH_O, _asyncio_Future_add_done_callback__doc__},
+    {"add_done_callback", (PyCFunction)_asyncio_Future_add_done_callback, METH_FASTCALL|METH_KEYWORDS, _asyncio_Future_add_done_callback__doc__},
+
+static PyObject *
+_asyncio_Future_add_done_callback_impl(FutureObj *self, PyObject *fn,
+                                       PyObject *context);
+
+static PyObject *
+_asyncio_Future_add_done_callback(FutureObj *self, PyObject *const *args, Py_ssize_t nargs, PyObject *kwnames)
+{
+    PyObject *return_value = NULL;
+    static const char * const _keywords[] = {"", "context", NULL};
+    static _PyArg_Parser _parser = {"O|$O:add_done_callback", _keywords, 0};
+    PyObject *fn;
+    PyObject *context = NULL;
+
+    if (!_PyArg_ParseStackAndKeywords(args, nargs, kwnames, &_parser,
+        &fn, &context)) {
+        goto exit;
+    }
+    return_value = _asyncio_Future_add_done_callback_impl(self, fn, context);
+
+exit:
+    return return_value;
+}
 
 PyDoc_STRVAR(_asyncio_Future_remove_done_callback__doc__,
 "remove_done_callback($self, fn, /)\n"
@@ -763,4 +786,4 @@ _asyncio__leave_task(PyObject *module, PyObject *const *args, Py_ssize_t nargs,
 exit:
     return return_value;
 }
-/*[clinic end generated code: output=616e814431893dcc input=a9049054013a1b77]*/
+/*[clinic end generated code: output=bcbaf1b2480f4aa9 input=a9049054013a1b77]*/
diff --git a/Modules/clinic/_contextvarsmodule.c.h b/Modules/clinic/_contextvarsmodule.c.h
new file mode 100644
index 0000000000..b1885e41c3
--- /dev/null
+++ b/Modules/clinic/_contextvarsmodule.c.h
@@ -0,0 +1,21 @@
+/*[clinic input]
+preserve
+[clinic start generated code]*/
+
+PyDoc_STRVAR(_contextvars_copy_context__doc__,
+"copy_context($module, /)\n"
+"--\n"
+"\n");
+
+#define _CONTEXTVARS_COPY_CONTEXT_METHODDEF    \
+    {"copy_context", (PyCFunction)_contextvars_copy_context, METH_NOARGS, _contextvars_copy_context__doc__},
+
+static PyObject *
+_contextvars_copy_context_impl(PyObject *module);
+
+static PyObject *
+_contextvars_copy_context(PyObject *module, PyObject *Py_UNUSED(ignored))
+{
+    return _contextvars_copy_context_impl(module);
+}
+/*[clinic end generated code: output=26e07024451baf52 input=a9049054013a1b77]*/
diff --git a/Modules/gcmodule.c b/Modules/gcmodule.c
index ea3c294dcf..8ba1093c02 100644
--- a/Modules/gcmodule.c
+++ b/Modules/gcmodule.c
@@ -24,6 +24,7 @@
 */
 
 #include "Python.h"
+#include "internal/context.h"
 #include "internal/mem.h"
 #include "internal/pystate.h"
 #include "frameobject.h"        /* for PyFrame_ClearFreeList */
@@ -790,6 +791,7 @@ clear_freelists(void)
     (void)PyDict_ClearFreeList();
     (void)PySet_ClearFreeList();
     (void)PyAsyncGen_ClearFreeLists();
+    (void)PyContext_ClearFreeList();
 }
 
 /* This is the main function.  Read this to understand how the
diff --git a/Objects/object.c b/Objects/object.c
index 62d7fbebf4..8cec6e2f12 100644
--- a/Objects/object.c
+++ b/Objects/object.c
@@ -3,6 +3,7 @@
 
 #include "Python.h"
 #include "internal/pystate.h"
+#include "internal/context.h"
 #include "frameobject.h"
 
 #ifdef __cplusplus
diff --git a/PCbuild/_contextvars.vcxproj b/PCbuild/_contextvars.vcxproj
new file mode 100644
index 0000000000..7418e86570
--- /dev/null
+++ b/PCbuild/_contextvars.vcxproj
@@ -0,0 +1,77 @@
+<?xml version="1.0" encoding="utf-8"?>
+<Project DefaultTargets="Build" ToolsVersion="4.0" xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
+  <ItemGroup Label="ProjectConfigurations">
+    <ProjectConfiguration Include="Debug|Win32">
+      <Configuration>Debug</Configuration>
+      <Platform>Win32</Platform>
+    </ProjectConfiguration>
+    <ProjectConfiguration Include="Debug|x64">
+      <Configuration>Debug</Configuration>
+      <Platform>x64</Platform>
+    </ProjectConfiguration>
+    <ProjectConfiguration Include="PGInstrument|Win32">
+      <Configuration>PGInstrument</Configuration>
+      <Platform>Win32</Platform>
+    </ProjectConfiguration>
+    <ProjectConfiguration Include="PGInstrument|x64">
+      <Configuration>PGInstrument</Configuration>
+      <Platform>x64</Platform>
+    </ProjectConfiguration>
+    <ProjectConfiguration Include="PGUpdate|Win32">
+      <Configuration>PGUpdate</Configuration>
+      <Platform>Win32</Platform>
+    </ProjectConfiguration>
+    <ProjectConfiguration Include="PGUpdate|x64">
+      <Configuration>PGUpdate</Configuration>
+      <Platform>x64</Platform>
+    </ProjectConfiguration>
+    <ProjectConfiguration Include="Release|Win32">
+      <Configuration>Release</Configuration>
+      <Platform>Win32</Platform>
+    </ProjectConfiguration>
+    <ProjectConfiguration Include="Release|x64">
+      <Configuration>Release</Configuration>
+      <Platform>x64</Platform>
+    </ProjectConfiguration>
+  </ItemGroup>
+  <PropertyGroup Label="Globals">
+    <ProjectGuid>{B8BF1D81-09DC-42D4-B406-4F868B33A89E}</ProjectGuid>
+    <RootNamespace>_contextvars</RootNamespace>
+    <Keyword>Win32Proj</Keyword>
+  </PropertyGroup>
+  <Import Project="python.props" />
+  <Import Project="$(VCTargetsPath)\Microsoft.Cpp.Default.props" />
+  <PropertyGroup Label="Configuration">
+    <ConfigurationType>DynamicLibrary</ConfigurationType>
+    <CharacterSet>NotSet</CharacterSet>
+  </PropertyGroup>
+  <Import Project="$(VCTargetsPath)\Microsoft.Cpp.props" />
+  <PropertyGroup>
+    <TargetExt>.pyd</TargetExt>
+  </PropertyGroup>
+  <ImportGroup Label="ExtensionSettings">
+  </ImportGroup>
+  <ImportGroup Label="PropertySheets">
+    <Import Project="$(UserRootDir)\Microsoft.Cpp.$(Platform).user.props" Condition="exists('$(UserRootDir)\Microsoft.Cpp.$(Platform).user.props')" Label="LocalAppDataPlatform" />
+    <Import Project="pyproject.props" />
+  </ImportGroup>
+  <PropertyGroup Label="UserMacros" />
+  <PropertyGroup>
+    <_ProjectFileVersion>10.0.30319.1</_ProjectFileVersion>
+  </PropertyGroup>
+  <ItemGroup>
+    <ClCompile Include="..\Modules\_contextvarsmodule.c" />
+  </ItemGroup>
+  <ItemGroup>
+    <ResourceCompile Include="..\PC\python_nt.rc" />
+  </ItemGroup>
+  <ItemGroup>
+    <ProjectReference Include="pythoncore.vcxproj">
+      <Project>{cf7ac3d1-e2df-41d2-bea6-1e2556cdea26}</Project>
+      <ReferenceOutputAssembly>false</ReferenceOutputAssembly>
+    </ProjectReference>
+  </ItemGroup>
+  <Import Project="$(VCTargetsPath)\Microsoft.Cpp.targets" />
+  <ImportGroup Label="ExtensionTargets">
+  </ImportGroup>
+</Project>
diff --git a/PCbuild/_contextvars.vcxproj.filters b/PCbuild/_contextvars.vcxproj.filters
new file mode 100644
index 0000000000..b3002b7ff6
--- /dev/null
+++ b/PCbuild/_contextvars.vcxproj.filters
@@ -0,0 +1,16 @@
+<?xml version="1.0" encoding="utf-8"?>
+<Project ToolsVersion="4.0" xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
+  <ItemGroup>
+    <ResourceCompile Include="..\PC\python_nt.rc" />
+  </ItemGroup>
+  <ItemGroup>
+    <Filter Include="Source Files">
+      <UniqueIdentifier>{7CBD8910-233D-4E9A-9164-9BA66C1F0E6D}</UniqueIdentifier>
+    </Filter>
+  </ItemGroup>
+  <ItemGroup>
+    <ClCompile Include="..\Modules\_contextvarsmodule.c">
+      <Filter>Source Files</Filter>
+    </ClCompile>
+  </ItemGroup>
+</Project>
diff --git a/PCbuild/_decimal.vcxproj b/PCbuild/_decimal.vcxproj
index b14f31093f..df9f600cda 100644
--- a/PCbuild/_decimal.vcxproj
+++ b/PCbuild/_decimal.vcxproj
@@ -121,4 +121,4 @@
   <Import Project="$(VCTargetsPath)\Microsoft.Cpp.targets" />
   <ImportGroup Label="ExtensionTargets">
   </ImportGroup>
-</Project>
\ No newline at end of file
+</Project>
diff --git a/PCbuild/pcbuild.proj b/PCbuild/pcbuild.proj
index 848b3b2877..5e341959bd 100644
--- a/PCbuild/pcbuild.proj
+++ b/PCbuild/pcbuild.proj
@@ -49,7 +49,7 @@
     <!-- pyshellext.dll -->
     <Projects Include="pyshellext.vcxproj" />
     <!-- Extension modules -->
-    <ExtensionModules Include="_asyncio;_ctypes;_decimal;_distutils_findvs;_elementtree;_msi;_multiprocessing;_overlapped;pyexpat;_queue;select;unicodedata;winsound" />
+    <ExtensionModules Include="_asyncio;_contextvars;_ctypes;_decimal;_distutils_findvs;_elementtree;_msi;_multiprocessing;_overlapped;pyexpat;_queue;select;unicodedata;winsound" />
     <!-- Extension modules that require external sources -->
     <ExternalModules Include="_bz2;_lzma;_sqlite3" />
     <!-- _ssl will build _socket as well, which may cause conflicts in parallel builds -->
diff --git a/PCbuild/pythoncore.vcxproj b/PCbuild/pythoncore.vcxproj
index bf2ce669f0..fbcd051213 100644
--- a/PCbuild/pythoncore.vcxproj
+++ b/PCbuild/pythoncore.vcxproj
@@ -94,6 +94,7 @@
     <ClInclude Include="..\Include\codecs.h" />
     <ClInclude Include="..\Include\compile.h" />
     <ClInclude Include="..\Include\complexobject.h" />
+    <ClInclude Include="..\Include\context.h" />
     <ClInclude Include="..\Include\datetime.h" />
     <ClInclude Include="..\Include\descrobject.h" />
     <ClInclude Include="..\Include\dictobject.h" />
@@ -112,7 +113,9 @@
     <ClInclude Include="..\Include\import.h" />
     <ClInclude Include="..\Include\internal\ceval.h" />
     <ClInclude Include="..\Include\internal\condvar.h" />
+    <ClInclude Include="..\Include\internal\context.h" />
     <ClInclude Include="..\Include\internal\gil.h" />
+    <ClInclude Include="..\Include\internal\hamt.h" />
     <ClInclude Include="..\Include\internal\mem.h" />
     <ClInclude Include="..\Include\internal\pystate.h" />
     <ClInclude Include="..\Include\internal\warnings.h" />
@@ -232,6 +235,7 @@
     <ClCompile Include="..\Modules\_blake2\blake2s_impl.c" />
     <ClCompile Include="..\Modules\_codecsmodule.c" />
     <ClCompile Include="..\Modules\_collectionsmodule.c" />
+    <ClCompile Include="..\Modules\_contextvarsmodule.c" />
     <ClCompile Include="..\Modules\_csv.c" />
     <ClCompile Include="..\Modules\_functoolsmodule.c" />
     <ClCompile Include="..\Modules\_heapqmodule.c" />
@@ -359,6 +363,7 @@
     <ClCompile Include="..\Python\ceval.c" />
     <ClCompile Include="..\Python\codecs.c" />
     <ClCompile Include="..\Python\compile.c" />
+    <ClCompile Include="..\Python\context.c" />
     <ClCompile Include="..\Python\dynamic_annotations.c" />
     <ClCompile Include="..\Python\dynload_win.c" />
     <ClCompile Include="..\Python\errors.c" />
@@ -373,6 +378,7 @@
     <ClCompile Include="..\Python\getplatform.c" />
     <ClCompile Include="..\Python\getversion.c" />
     <ClCompile Include="..\Python\graminit.c" />
+    <ClCompile Include="..\Python\hamt.c" />
     <ClCompile Include="..\Python\import.c" />
     <ClCompile Include="..\Python\importdl.c" />
     <ClCompile Include="..\Python\marshal.c" />
diff --git a/PCbuild/pythoncore.vcxproj.filters b/PCbuild/pythoncore.vcxproj.filters
index 13600cb5c9..a10686c194 100644
--- a/PCbuild/pythoncore.vcxproj.filters
+++ b/PCbuild/pythoncore.vcxproj.filters
@@ -81,6 +81,9 @@
     <ClInclude Include="..\Include\complexobject.h">
       <Filter>Include</Filter>
     </ClInclude>
+    <ClInclude Include="..\Include\context.h">
+      <Filter>Include</Filter>
+    </ClInclude>
     <ClInclude Include="..\Include\datetime.h">
       <Filter>Include</Filter>
     </ClInclude>
@@ -135,9 +138,15 @@
     <ClInclude Include="..\Include\internal\condvar.h">
       <Filter>Include</Filter>
     </ClInclude>
+    <ClInclude Include="..\Include\internal\context.h">
+      <Filter>Include</Filter>
+    </ClInclude>
     <ClInclude Include="..\Include\internal\gil.h">
       <Filter>Include</Filter>
     </ClInclude>
+    <ClInclude Include="..\Include\internal\hamt.h">
+      <Filter>Include</Filter>
+    </ClInclude>
     <ClInclude Include="..\Include\internal\mem.h">
       <Filter>Include</Filter>
     </ClInclude>
@@ -842,6 +851,9 @@
     <ClCompile Include="..\Python\compile.c">
       <Filter>Python</Filter>
     </ClCompile>
+    <ClCompile Include="..\Python\context.h">
+      <Filter>Python</Filter>
+    </ClCompile>
     <ClCompile Include="..\Python\dynamic_annotations.c">
       <Filter>Python</Filter>
     </ClCompile>
@@ -884,6 +896,9 @@
     <ClCompile Include="..\Python\graminit.c">
       <Filter>Python</Filter>
     </ClCompile>
+    <ClCompile Include="..\Python\hamt.h">
+      <Filter>Python</Filter>
+    </ClCompile>
     <ClCompile Include="..\Python\import.c">
       <Filter>Python</Filter>
     </ClCompile>
@@ -998,6 +1013,9 @@
     <ClCompile Include="..\Modules\_asynciomodule.c">
       <Filter>Modules</Filter>
     </ClCompile>
+    <ClCompile Include="..\Modules\_contextvarsmodule.c">
+      <Filter>Modules</Filter>
+    </ClCompile>
     <ClCompile Include="$(zlibDir)\adler32.c">
       <Filter>Modules\zlib</Filter>
     </ClCompile>
diff --git a/Python/clinic/context.c.h b/Python/clinic/context.c.h
new file mode 100644
index 0000000000..dcf4c214e1
--- /dev/null
+++ b/Python/clinic/context.c.h
@@ -0,0 +1,146 @@
+/*[clinic input]
+preserve
+[clinic start generated code]*/
+
+PyDoc_STRVAR(_contextvars_Context_get__doc__,
+"get($self, key, default=None, /)\n"
+"--\n"
+"\n");
+
+#define _CONTEXTVARS_CONTEXT_GET_METHODDEF    \
+    {"get", (PyCFunction)_contextvars_Context_get, METH_FASTCALL, _contextvars_Context_get__doc__},
+
+static PyObject *
+_contextvars_Context_get_impl(PyContext *self, PyObject *key,
+                              PyObject *default_value);
+
+static PyObject *
+_contextvars_Context_get(PyContext *self, PyObject *const *args, Py_ssize_t nargs)
+{
+    PyObject *return_value = NULL;
+    PyObject *key;
+    PyObject *default_value = Py_None;
+
+    if (!_PyArg_UnpackStack(args, nargs, "get",
+        1, 2,
+        &key, &default_value)) {
+        goto exit;
+    }
+    return_value = _contextvars_Context_get_impl(self, key, default_value);
+
+exit:
+    return return_value;
+}
+
+PyDoc_STRVAR(_contextvars_Context_items__doc__,
+"items($self, /)\n"
+"--\n"
+"\n");
+
+#define _CONTEXTVARS_CONTEXT_ITEMS_METHODDEF    \
+    {"items", (PyCFunction)_contextvars_Context_items, METH_NOARGS, _contextvars_Context_items__doc__},
+
+static PyObject *
+_contextvars_Context_items_impl(PyContext *self);
+
+static PyObject *
+_contextvars_Context_items(PyContext *self, PyObject *Py_UNUSED(ignored))
+{
+    return _contextvars_Context_items_impl(self);
+}
+
+PyDoc_STRVAR(_contextvars_Context_keys__doc__,
+"keys($self, /)\n"
+"--\n"
+"\n");
+
+#define _CONTEXTVARS_CONTEXT_KEYS_METHODDEF    \
+    {"keys", (PyCFunction)_contextvars_Context_keys, METH_NOARGS, _contextvars_Context_keys__doc__},
+
+static PyObject *
+_contextvars_Context_keys_impl(PyContext *self);
+
+static PyObject *
+_contextvars_Context_keys(PyContext *self, PyObject *Py_UNUSED(ignored))
+{
+    return _contextvars_Context_keys_impl(self);
+}
+
+PyDoc_STRVAR(_contextvars_Context_values__doc__,
+"values($self, /)\n"
+"--\n"
+"\n");
+
+#define _CONTEXTVARS_CONTEXT_VALUES_METHODDEF    \
+    {"values", (PyCFunction)_contextvars_Context_values, METH_NOARGS, _contextvars_Context_values__doc__},
+
+static PyObject *
+_contextvars_Context_values_impl(PyContext *self);
+
+static PyObject *
+_contextvars_Context_values(PyContext *self, PyObject *Py_UNUSED(ignored))
+{
+    return _contextvars_Context_values_impl(self);
+}
+
+PyDoc_STRVAR(_contextvars_Context_copy__doc__,
+"copy($self, /)\n"
+"--\n"
+"\n");
+
+#define _CONTEXTVARS_CONTEXT_COPY_METHODDEF    \
+    {"copy", (PyCFunction)_contextvars_Context_copy, METH_NOARGS, _contextvars_Context_copy__doc__},
+
+static PyObject *
+_contextvars_Context_copy_impl(PyContext *self);
+
+static PyObject *
+_contextvars_Context_copy(PyContext *self, PyObject *Py_UNUSED(ignored))
+{
+    return _contextvars_Context_copy_impl(self);
+}
+
+PyDoc_STRVAR(_contextvars_ContextVar_get__doc__,
+"get($self, default=None, /)\n"
+"--\n"
+"\n");
+
+#define _CONTEXTVARS_CONTEXTVAR_GET_METHODDEF    \
+    {"get", (PyCFunction)_contextvars_ContextVar_get, METH_FASTCALL, _contextvars_ContextVar_get__doc__},
+
+static PyObject *
+_contextvars_ContextVar_get_impl(PyContextVar *self, PyObject *default_value);
+
+static PyObject *
+_contextvars_ContextVar_get(PyContextVar *self, PyObject *const *args, Py_ssize_t nargs)
+{
+    PyObject *return_value = NULL;
+    PyObject *default_value = NULL;
+
+    if (!_PyArg_UnpackStack(args, nargs, "get",
+        0, 1,
+        &default_value)) {
+        goto exit;
+    }
+    return_value = _contextvars_ContextVar_get_impl(self, default_value);
+
+exit:
+    return return_value;
+}
+
+PyDoc_STRVAR(_contextvars_ContextVar_set__doc__,
+"set($self, value, /)\n"
+"--\n"
+"\n");
+
+#define _CONTEXTVARS_CONTEXTVAR_SET_METHODDEF    \
+    {"set", (PyCFunction)_contextvars_ContextVar_set, METH_O, _contextvars_ContextVar_set__doc__},
+
+PyDoc_STRVAR(_contextvars_ContextVar_reset__doc__,
+"reset($self, token, /)\n"
+"--\n"
+"\n");
+
+#define _CONTEXTVARS_CONTEXTVAR_RESET_METHODDEF    \
+    {"reset", (PyCFunction)_contextvars_ContextVar_reset, METH_O, _contextvars_ContextVar_reset__doc__},
+/*[clinic end generated code: output=d9a675e3a52a14fc input=a9049054013a1b77]*/
diff --git a/Python/context.c b/Python/context.c
new file mode 100644
index 0000000000..2f1d0f5c34
--- /dev/null
+++ b/Python/context.c
@@ -0,0 +1,1220 @@
+#include "Python.h"
+
+#include "structmember.h"
+#include "internal/pystate.h"
+#include "internal/context.h"
+#include "internal/hamt.h"
+
+
+#define CONTEXT_FREELIST_MAXLEN 255
+static PyContext *ctx_freelist = NULL;
+static Py_ssize_t ctx_freelist_len = 0;
+
+
+#include "clinic/context.c.h"
+/*[clinic input]
+module _contextvars
+[clinic start generated code]*/
+/*[clinic end generated code: output=da39a3ee5e6b4b0d input=a0955718c8b8cea6]*/
+
+
+/////////////////////////// Context API
+
+
+static PyContext *
+context_new_empty(void);
+
+static PyContext *
+context_new_from_vars(PyHamtObject *vars);
+
+static inline PyContext *
+context_get(void);
+
+static PyContextToken *
+token_new(PyContext *ctx, PyContextVar *var, PyObject *val);
+
+static PyContextVar *
+contextvar_new(PyObject *name, PyObject *def);
+
+static int
+contextvar_set(PyContextVar *var, PyObject *val);
+
+static int
+contextvar_del(PyContextVar *var);
+
+
+PyObject *
+_PyContext_NewHamtForTests(void)
+{
+    return (PyObject *)_PyHamt_New();
+}
+
+
+PyContext *
+PyContext_New(void)
+{
+    return context_new_empty();
+}
+
+
+PyContext *
+PyContext_Copy(PyContext * ctx)
+{
+    return context_new_from_vars(ctx->ctx_vars);
+}
+
+
+PyContext *
+PyContext_CopyCurrent(void)
+{
+    PyContext *ctx = context_get();
+    if (ctx == NULL) {
+        return NULL;
+    }
+
+    return context_new_from_vars(ctx->ctx_vars);
+}
+
+
+int
+PyContext_Enter(PyContext *ctx)
+{
+    if (ctx->ctx_entered) {
+        PyErr_Format(PyExc_RuntimeError,
+                     "cannot enter context: %R is already entered", ctx);
+        return -1;
+    }
+
+    PyThreadState *ts = PyThreadState_Get();
+
+    ctx->ctx_prev = (PyContext *)ts->context;  /* borrow */
+    ctx->ctx_entered = 1;
+
+    Py_INCREF(ctx);
+    ts->context = (PyObject *)ctx;
+    ts->context_ver++;
+
+    return 0;
+}
+
+
+int
+PyContext_Exit(PyContext *ctx)
+{
+    if (!ctx->ctx_entered) {
+        PyErr_Format(PyExc_RuntimeError,
+                     "cannot exit context: %R has not been entered", ctx);
+        return -1;
+    }
+
+    PyThreadState *ts = PyThreadState_Get();
+
+    if (ts->context != (PyObject *)ctx) {
+        /* Can only happen if someone misuses the C API */
+        PyErr_SetString(PyExc_RuntimeError,
+                        "cannot exit context: thread state references "
+                        "a different context object");
+        return -1;
+    }
+
+    Py_SETREF(ts->context, (PyObject *)ctx->ctx_prev);
+    ts->context_ver++;
+
+    ctx->ctx_prev = NULL;
+    ctx->ctx_entered = 0;
+
+    return 0;
+}
+
+
+PyContextVar *
+PyContextVar_New(const char *name, PyObject *def)
+{
+    PyObject *pyname = PyUnicode_FromString(name);
+    if (pyname == NULL) {
+        return NULL;
+    }
+    return contextvar_new(pyname, def);
+}
+
+
+int
+PyContextVar_Get(PyContextVar *var, PyObject *def, PyObject **val)
+{
+    assert(PyContextVar_CheckExact(var));
+
+    PyThreadState *ts = PyThreadState_Get();
+    if (ts->context == NULL) {
+        goto not_found;
+    }
+
+    if (var->var_cached != NULL &&
+            var->var_cached_tsid == ts->id &&
+            var->var_cached_tsver == ts->context_ver)
+    {
+        *val = var->var_cached;
+        goto found;
+    }
+
+    assert(PyContext_CheckExact(ts->context));
+    PyHamtObject *vars = ((PyContext *)ts->context)->ctx_vars;
+
+    PyObject *found = NULL;
+    int res = _PyHamt_Find(vars, (PyObject*)var, &found);
+    if (res < 0) {
+        goto error;
+    }
+    if (res == 1) {
+        assert(found != NULL);
+        var->var_cached = found;  /* borrow */
+        var->var_cached_tsid = ts->id;
+        var->var_cached_tsver = ts->context_ver;
+
+        *val = found;
+        goto found;
+    }
+
+not_found:
+    if (def == NULL) {
+        if (var->var_default != NULL) {
+            *val = var->var_default;
+            goto found;
+        }
+
+        *val = NULL;
+        goto found;
+    }
+    else {
+        *val = def;
+        goto found;
+   }
+
+found:
+    Py_XINCREF(*val);
+    return 0;
+
+error:
+    *val = NULL;
+    return -1;
+}
+
+
+PyContextToken *
+PyContextVar_Set(PyContextVar *var, PyObject *val)
+{
+    if (!PyContextVar_CheckExact(var)) {
+        PyErr_SetString(
+            PyExc_TypeError, "an instance of ContextVar was expected");
+        return NULL;
+    }
+
+    PyContext *ctx = context_get();
+    if (ctx == NULL) {
+        return NULL;
+    }
+
+    PyObject *old_val = NULL;
+    int found = _PyHamt_Find(ctx->ctx_vars, (PyObject *)var, &old_val);
+    if (found < 0) {
+        return NULL;
+    }
+
+    Py_XINCREF(old_val);
+    PyContextToken *tok = token_new(ctx, var, old_val);
+    Py_XDECREF(old_val);
+
+    if (contextvar_set(var, val)) {
+        Py_DECREF(tok);
+        return NULL;
+    }
+
+    return tok;
+}
+
+
+int
+PyContextVar_Reset(PyContextVar *var, PyContextToken *tok)
+{
+    if (tok->tok_used) {
+        PyErr_Format(PyExc_RuntimeError,
+                     "%R has already been used once", tok);
+        return -1;
+    }
+
+    if (var != tok->tok_var) {
+        PyErr_Format(PyExc_ValueError,
+                     "%R was created by a different ContextVar", tok);
+        return -1;
+    }
+
+    PyContext *ctx = context_get();
+    if (ctx != tok->tok_ctx) {
+        PyErr_Format(PyExc_ValueError,
+                     "%R was created in a different Context", tok);
+        return -1;
+    }
+
+    tok->tok_used = 1;
+
+    if (tok->tok_oldval == NULL) {
+        return contextvar_del(var);
+    }
+    else {
+        return contextvar_set(var, tok->tok_oldval);
+    }
+}
+
+
+/////////////////////////// PyContext
+
+/*[clinic input]
+class _contextvars.Context "PyContext *" "&PyContext_Type"
+[clinic start generated code]*/
+/*[clinic end generated code: output=da39a3ee5e6b4b0d input=bdf87f8e0cb580e8]*/
+
+
+static inline PyContext *
+_context_alloc(void)
+{
+    PyContext *ctx;
+    if (ctx_freelist_len) {
+        ctx_freelist_len--;
+        ctx = ctx_freelist;
+        ctx_freelist = (PyContext *)ctx->ctx_weakreflist;
+        ctx->ctx_weakreflist = NULL;
+        _Py_NewReference((PyObject *)ctx);
+    }
+    else {
+        ctx = PyObject_GC_New(PyContext, &PyContext_Type);
+        if (ctx == NULL) {
+            return NULL;
+        }
+    }
+
+    ctx->ctx_vars = NULL;
+    ctx->ctx_prev = NULL;
+    ctx->ctx_entered = 0;
+    ctx->ctx_weakreflist = NULL;
+
+    return ctx;
+}
+
+
+static PyContext *
+context_new_empty(void)
+{
+    PyContext *ctx = _context_alloc();
+    if (ctx == NULL) {
+        return NULL;
+    }
+
+    ctx->ctx_vars = _PyHamt_New();
+    if (ctx->ctx_vars == NULL) {
+        Py_DECREF(ctx);
+        return NULL;
+    }
+
+    _PyObject_GC_TRACK(ctx);
+    return ctx;
+}
+
+
+static PyContext *
+context_new_from_vars(PyHamtObject *vars)
+{
+    PyContext *ctx = _context_alloc();
+    if (ctx == NULL) {
+        return NULL;
+    }
+
+    Py_INCREF(vars);
+    ctx->ctx_vars = vars;
+
+    _PyObject_GC_TRACK(ctx);
+    return ctx;
+}
+
+
+static inline PyContext *
+context_get(void)
+{
+    PyThreadState *ts = PyThreadState_Get();
+    PyContext *current_ctx = (PyContext *)ts->context;
+    if (current_ctx == NULL) {
+        current_ctx = context_new_empty();
+        if (current_ctx == NULL) {
+            return NULL;
+        }
+        ts->context = (PyObject *)current_ctx;
+    }
+    return current_ctx;
+}
+
+static int
+context_check_key_type(PyObject *key)
+{
+    if (!PyContextVar_CheckExact(key)) {
+        // abort();
+        PyErr_Format(PyExc_TypeError,
+                     "a ContextVar key was expected, got %R", key);
+        return -1;
+    }
+    return 0;
+}
+
+static PyObject *
+context_tp_new(PyTypeObject *type, PyObject *args, PyObject *kwds)
+{
+    if (PyTuple_Size(args) || (kwds != NULL && PyDict_Size(kwds))) {
+        PyErr_SetString(
+            PyExc_TypeError, "Context() does not accept any arguments");
+        return NULL;
+    }
+    return (PyObject *)PyContext_New();
+}
+
+static int
+context_tp_clear(PyContext *self)
+{
+    Py_CLEAR(self->ctx_prev);
+    Py_CLEAR(self->ctx_vars);
+    return 0;
+}
+
+static int
+context_tp_traverse(PyContext *self, visitproc visit, void *arg)
+{
+    Py_VISIT(self->ctx_prev);
+    Py_VISIT(self->ctx_vars);
+    return 0;
+}
+
+static void
+context_tp_dealloc(PyContext *self)
+{
+    _PyObject_GC_UNTRACK(self);
+
+    if (self->ctx_weakreflist != NULL) {
+        PyObject_ClearWeakRefs((PyObject*)self);
+    }
+    (void)context_tp_clear(self);
+
+    if (ctx_freelist_len < CONTEXT_FREELIST_MAXLEN) {
+        ctx_freelist_len++;
+        self->ctx_weakreflist = (PyObject *)ctx_freelist;
+        ctx_freelist = self;
+    }
+    else {
+        Py_TYPE(self)->tp_free(self);
+    }
+}
+
+static PyObject *
+context_tp_iter(PyContext *self)
+{
+    return _PyHamt_NewIterKeys(self->ctx_vars);
+}
+
+static PyObject *
+context_tp_richcompare(PyObject *v, PyObject *w, int op)
+{
+    if (!PyContext_CheckExact(v) || !PyContext_CheckExact(w) ||
+            (op != Py_EQ && op != Py_NE))
+    {
+        Py_RETURN_NOTIMPLEMENTED;
+    }
+
+    int res = _PyHamt_Eq(
+        ((PyContext *)v)->ctx_vars, ((PyContext *)w)->ctx_vars);
+    if (res < 0) {
+        return NULL;
+    }
+
+    if (op == Py_NE) {
+        res = !res;
+    }
+
+    if (res) {
+        Py_RETURN_TRUE;
+    }
+    else {
+        Py_RETURN_FALSE;
+    }
+}
+
+static Py_ssize_t
+context_tp_len(PyContext *self)
+{
+    return _PyHamt_Len(self->ctx_vars);
+}
+
+static PyObject *
+context_tp_subscript(PyContext *self, PyObject *key)
+{
+    if (context_check_key_type(key)) {
+        return NULL;
+    }
+    PyObject *val = NULL;
+    int found = _PyHamt_Find(self->ctx_vars, key, &val);
+    if (found < 0) {
+        return NULL;
+    }
+    if (found == 0) {
+        PyErr_SetObject(PyExc_KeyError, key);
+        return NULL;
+    }
+    Py_INCREF(val);
+    return val;
+}
+
+static int
+context_tp_contains(PyContext *self, PyObject *key)
+{
+    if (context_check_key_type(key)) {
+        return -1;
+    }
+    PyObject *val = NULL;
+    return _PyHamt_Find(self->ctx_vars, key, &val);
+}
+
+
+/*[clinic input]
+_contextvars.Context.get
+    key: object
+    default: object = None
+    /
+[clinic start generated code]*/
+
+static PyObject *
+_contextvars_Context_get_impl(PyContext *self, PyObject *key,
+                              PyObject *default_value)
+/*[clinic end generated code: output=0c54aa7664268189 input=8d4c33c8ecd6d769]*/
+{
+    if (context_check_key_type(key)) {
+        return NULL;
+    }
+
+    PyObject *val = NULL;
+    int found = _PyHamt_Find(self->ctx_vars, key, &val);
+    if (found < 0) {
+        return NULL;
+    }
+    if (found == 0) {
+        Py_INCREF(default_value);
+        return default_value;
+    }
+    Py_INCREF(val);
+    return val;
+}
+
+
+/*[clinic input]
+_contextvars.Context.items
+[clinic start generated code]*/
+
+static PyObject *
+_contextvars_Context_items_impl(PyContext *self)
+/*[clinic end generated code: output=fa1655c8a08502af input=2d570d1455004979]*/
+{
+    return _PyHamt_NewIterItems(self->ctx_vars);
+}
+
+
+/*[clinic input]
+_contextvars.Context.keys
+[clinic start generated code]*/
+
+static PyObject *
+_contextvars_Context_keys_impl(PyContext *self)
+/*[clinic end generated code: output=177227c6b63ec0e2 input=13005e142fbbf37d]*/
+{
+    return _PyHamt_NewIterKeys(self->ctx_vars);
+}
+
+
+/*[clinic input]
+_contextvars.Context.values
+[clinic start generated code]*/
+
+static PyObject *
+_contextvars_Context_values_impl(PyContext *self)
+/*[clinic end generated code: output=d286dabfc8db6dde input=c2cbc40a4470e905]*/
+{
+    return _PyHamt_NewIterValues(self->ctx_vars);
+}
+
+
+/*[clinic input]
+_contextvars.Context.copy
+[clinic start generated code]*/
+
+static PyObject *
+_contextvars_Context_copy_impl(PyContext *self)
+/*[clinic end generated code: output=30ba8896c4707a15 input=3e3fd72d598653ab]*/
+{
+    return (PyObject *)context_new_from_vars(self->ctx_vars);
+}
+
+
+static PyObject *
+context_run(PyContext *self, PyObject *const *args,
+            Py_ssize_t nargs, PyObject *kwnames)
+{
+    if (nargs < 1) {
+        PyErr_SetString(PyExc_TypeError,
+                        "run() missing 1 required positional argument");
+        return NULL;
+    }
+
+    if (PyContext_Enter(self)) {
+        return NULL;
+    }
+
+    PyObject *call_result = _PyObject_FastCallKeywords(
+        args[0], args + 1, nargs - 1, kwnames);
+
+    if (PyContext_Exit(self)) {
+        return NULL;
+    }
+
+    return call_result;
+}
+
+
+static PyMethodDef PyContext_methods[] = {
+    _CONTEXTVARS_CONTEXT_GET_METHODDEF
+    _CONTEXTVARS_CONTEXT_ITEMS_METHODDEF
+    _CONTEXTVARS_CONTEXT_KEYS_METHODDEF
+    _CONTEXTVARS_CONTEXT_VALUES_METHODDEF
+    _CONTEXTVARS_CONTEXT_COPY_METHODDEF
+    {"run", (PyCFunction)context_run, METH_FASTCALL | METH_KEYWORDS, NULL},
+    {NULL, NULL}
+};
+
+static PySequenceMethods PyContext_as_sequence = {
+    0,                                   /* sq_length */
+    0,                                   /* sq_concat */
+    0,                                   /* sq_repeat */
+    0,                                   /* sq_item */
+    0,                                   /* sq_slice */
+    0,                                   /* sq_ass_item */
+    0,                                   /* sq_ass_slice */
+    (objobjproc)context_tp_contains,     /* sq_contains */
+    0,                                   /* sq_inplace_concat */
+    0,                                   /* sq_inplace_repeat */
+};
+
+static PyMappingMethods PyContext_as_mapping = {
+    (lenfunc)context_tp_len,             /* mp_length */
+    (binaryfunc)context_tp_subscript,    /* mp_subscript */
+};
+
+PyTypeObject PyContext_Type = {
+    PyVarObject_HEAD_INIT(&PyType_Type, 0)
+    "Context",
+    sizeof(PyContext),
+    .tp_methods = PyContext_methods,
+    .tp_as_mapping = &PyContext_as_mapping,
+    .tp_as_sequence = &PyContext_as_sequence,
+    .tp_iter = (getiterfunc)context_tp_iter,
+    .tp_dealloc = (destructor)context_tp_dealloc,
+    .tp_getattro = PyObject_GenericGetAttr,
+    .tp_flags = Py_TPFLAGS_DEFAULT | Py_TPFLAGS_HAVE_GC,
+    .tp_richcompare = context_tp_richcompare,
+    .tp_traverse = (traverseproc)context_tp_traverse,
+    .tp_clear = (inquiry)context_tp_clear,
+    .tp_new = context_tp_new,
+    .tp_weaklistoffset = offsetof(PyContext, ctx_weakreflist),
+    .tp_hash = PyObject_HashNotImplemented,
+};
+
+
+/////////////////////////// ContextVar
+
+
+static int
+contextvar_set(PyContextVar *var, PyObject *val)
+{
+    var->var_cached = NULL;
+    PyThreadState *ts = PyThreadState_Get();
+
+    PyContext *ctx = context_get();
+    if (ctx == NULL) {
+        return -1;
+    }
+
+    PyHamtObject *new_vars = _PyHamt_Assoc(
+        ctx->ctx_vars, (PyObject *)var, val);
+    if (new_vars == NULL) {
+        return -1;
+    }
+
+    Py_SETREF(ctx->ctx_vars, new_vars);
+
+    var->var_cached = val;  /* borrow */
+    var->var_cached_tsid = ts->id;
+    var->var_cached_tsver = ts->context_ver;
+    return 0;
+}
+
+static int
+contextvar_del(PyContextVar *var)
+{
+    var->var_cached = NULL;
+
+    PyContext *ctx = context_get();
+    if (ctx == NULL) {
+        return -1;
+    }
+
+    PyHamtObject *vars = ctx->ctx_vars;
+    PyHamtObject *new_vars = _PyHamt_Without(vars, (PyObject *)var);
+    if (new_vars == NULL) {
+        return -1;
+    }
+
+    if (vars == new_vars) {
+        Py_DECREF(new_vars);
+        PyErr_SetObject(PyExc_LookupError, (PyObject *)var);
+        return -1;
+    }
+
+    Py_SETREF(ctx->ctx_vars, new_vars);
+    return 0;
+}
+
+static Py_hash_t
+contextvar_generate_hash(void *addr, PyObject *name)
+{
+    /* Take hash of `name` and XOR it with the object's addr.
+
+       The structure of the tree is encoded in objects' hashes, which
+       means that sufficiently similar hashes would result in tall trees
+       with many Collision nodes.  Which would, in turn, result in slower
+       get and set operations.
+
+       The XORing helps to ensure that:
+
+       (1) sequentially allocated ContextVar objects have
+           different hashes;
+
+       (2) context variables with equal names have
+           different hashes.
+    */
+
+    Py_hash_t name_hash = PyObject_Hash(name);
+    if (name_hash == -1) {
+        return -1;
+    }
+
+    Py_hash_t res = _Py_HashPointer(addr) ^ name_hash;
+    return res == -1 ? -2 : res;
+}
+
+static PyContextVar *
+contextvar_new(PyObject *name, PyObject *def)
+{
+    if (!PyUnicode_Check(name)) {
+        PyErr_SetString(PyExc_TypeError,
+                        "context variable name must be a str");
+        return NULL;
+    }
+
+    PyContextVar *var = PyObject_GC_New(PyContextVar, &PyContextVar_Type);
+    if (var == NULL) {
+        return NULL;
+    }
+
+    var->var_hash = contextvar_generate_hash(var, name);
+    if (var->var_hash == -1) {
+        Py_DECREF(var);
+        return NULL;
+    }
+
+    Py_INCREF(name);
+    var->var_name = name;
+
+    Py_XINCREF(def);
+    var->var_default = def;
+
+    var->var_cached = NULL;
+    var->var_cached_tsid = 0;
+    var->var_cached_tsver = 0;
+
+    if (_PyObject_GC_IS_TRACKED(name) ||
+            (def != NULL && _PyObject_GC_IS_TRACKED(def)))
+    {
+        PyObject_GC_Track(var);
+    }
+    return var;
+}
+
+
+/*[clinic input]
+class _contextvars.ContextVar "PyContextVar *" "&PyContextVar_Type"
+[clinic start generated code]*/
+/*[clinic end generated code: output=da39a3ee5e6b4b0d input=445da935fa8883c3]*/
+
+
+static PyObject *
+contextvar_tp_new(PyTypeObject *type, PyObject *args, PyObject *kwds)
+{
+    static char *kwlist[] = {"", "default", NULL};
+    PyObject *name;
+    PyObject *def = NULL;
+
+    if (!PyArg_ParseTupleAndKeywords(
+            args, kwds, "O|$O:ContextVar", kwlist, &name, &def))
+    {
+        return NULL;
+    }
+
+    return (PyObject *)contextvar_new(name, def);
+}
+
+static int
+contextvar_tp_clear(PyContextVar *self)
+{
+    Py_CLEAR(self->var_name);
+    Py_CLEAR(self->var_default);
+    self->var_cached = NULL;
+    self->var_cached_tsid = 0;
+    self->var_cached_tsver = 0;
+    return 0;
+}
+
+static int
+contextvar_tp_traverse(PyContextVar *self, visitproc visit, void *arg)
+{
+    Py_VISIT(self->var_name);
+    Py_VISIT(self->var_default);
+    return 0;
+}
+
+static void
+contextvar_tp_dealloc(PyContextVar *self)
+{
+    PyObject_GC_UnTrack(self);
+    (void)contextvar_tp_clear(self);
+    Py_TYPE(self)->tp_free(self);
+}
+
+static Py_hash_t
+contextvar_tp_hash(PyContextVar *self)
+{
+    return self->var_hash;
+}
+
+static PyObject *
+contextvar_tp_repr(PyContextVar *self)
+{
+    _PyUnicodeWriter writer;
+
+    _PyUnicodeWriter_Init(&writer);
+
+    if (_PyUnicodeWriter_WriteASCIIString(
+            &writer, "<ContextVar name=", 17) < 0)
+    {
+        goto error;
+    }
+
+    PyObject *name = PyObject_Repr(self->var_name);
+    if (name == NULL) {
+        goto error;
+    }
+    if (_PyUnicodeWriter_WriteStr(&writer, name) < 0) {
+        Py_DECREF(name);
+        goto error;
+    }
+    Py_DECREF(name);
+
+    if (self->var_default != NULL) {
+        if (_PyUnicodeWriter_WriteASCIIString(&writer, " default=", 9) < 0) {
+            goto error;
+        }
+
+        PyObject *def = PyObject_Repr(self->var_default);
+        if (def == NULL) {
+            goto error;
+        }
+        if (_PyUnicodeWriter_WriteStr(&writer, def) < 0) {
+            Py_DECREF(def);
+            goto error;
+        }
+        Py_DECREF(def);
+    }
+
+    PyObject *addr = PyUnicode_FromFormat(" at %p>", self);
+    if (addr == NULL) {
+        goto error;
+    }
+    if (_PyUnicodeWriter_WriteStr(&writer, addr) < 0) {
+        Py_DECREF(addr);
+        goto error;
+    }
+    Py_DECREF(addr);
+
+    return _PyUnicodeWriter_Finish(&writer);
+
+error:
+    _PyUnicodeWriter_Dealloc(&writer);
+    return NULL;
+}
+
+
+/*[clinic input]
+_contextvars.ContextVar.get
+    default: object = NULL
+    /
+[clinic start generated code]*/
+
+static PyObject *
+_contextvars_ContextVar_get_impl(PyContextVar *self, PyObject *default_value)
+/*[clinic end generated code: output=0746bd0aa2ced7bf input=8d002b02eebbb247]*/
+{
+    if (!PyContextVar_CheckExact(self)) {
+        PyErr_SetString(
+            PyExc_TypeError, "an instance of ContextVar was expected");
+        return NULL;
+    }
+
+    PyObject *val;
+    if (PyContextVar_Get(self, default_value, &val) < 0) {
+        return NULL;
+    }
+
+    if (val == NULL) {
+        PyErr_SetObject(PyExc_LookupError, (PyObject *)self);
+        return NULL;
+    }
+
+    return val;
+}
+
+/*[clinic input]
+_contextvars.ContextVar.set
+    value: object
+    /
+[clinic start generated code]*/
+
+static PyObject *
+_contextvars_ContextVar_set(PyContextVar *self, PyObject *value)
+/*[clinic end generated code: output=446ed5e820d6d60b input=a2d88f57c6d86f7c]*/
+{
+    return (PyObject *)PyContextVar_Set(self, value);
+}
+
+/*[clinic input]
+_contextvars.ContextVar.reset
+    token: object
+    /
+[clinic start generated code]*/
+
+static PyObject *
+_contextvars_ContextVar_reset(PyContextVar *self, PyObject *token)
+/*[clinic end generated code: output=d4ee34d0742d62ee input=4c871b6f1f31a65f]*/
+{
+    if (!PyContextToken_CheckExact(token)) {
+        PyErr_Format(PyExc_TypeError,
+                     "expected an instance of Token, got %R", token);
+        return NULL;
+    }
+
+    if (PyContextVar_Reset(self, (PyContextToken *)token)) {
+        return NULL;
+    }
+
+    Py_RETURN_NONE;
+}
+
+
+static PyObject *
+contextvar_cls_getitem(PyObject *self, PyObject *args)
+{
+    Py_RETURN_NONE;
+}
+
+
+static PyMethodDef PyContextVar_methods[] = {
+    _CONTEXTVARS_CONTEXTVAR_GET_METHODDEF
+    _CONTEXTVARS_CONTEXTVAR_SET_METHODDEF
+    _CONTEXTVARS_CONTEXTVAR_RESET_METHODDEF
+    {"__class_getitem__", contextvar_cls_getitem,
+        METH_VARARGS | METH_STATIC, NULL},
+    {NULL, NULL}
+};
+
+PyTypeObject PyContextVar_Type = {
+    PyVarObject_HEAD_INIT(&PyType_Type, 0)
+    "ContextVar",
+    sizeof(PyContextVar),
+    .tp_methods = PyContextVar_methods,
+    .tp_dealloc = (destructor)contextvar_tp_dealloc,
+    .tp_getattro = PyObject_GenericGetAttr,
+    .tp_flags = Py_TPFLAGS_DEFAULT | Py_TPFLAGS_HAVE_GC,
+    .tp_traverse = (traverseproc)contextvar_tp_traverse,
+    .tp_clear = (inquiry)contextvar_tp_clear,
+    .tp_new = contextvar_tp_new,
+    .tp_free = PyObject_GC_Del,
+    .tp_hash = (hashfunc)contextvar_tp_hash,
+    .tp_repr = (reprfunc)contextvar_tp_repr,
+};
+
+
+/////////////////////////// Token
+
+static PyObject * get_token_missing(void);
+
+
+/*[clinic input]
+class _contextvars.Token "PyContextToken *" "&PyContextToken_Type"
+[clinic start generated code]*/
+/*[clinic end generated code: output=da39a3ee5e6b4b0d input=338a5e2db13d3f5b]*/
+
+
+static PyObject *
+token_tp_new(PyTypeObject *type, PyObject *args, PyObject *kwds)
+{
+    PyErr_SetString(PyExc_RuntimeError,
+                    "Tokens can only be created by ContextVars");
+    return NULL;
+}
+
+static int
+token_tp_clear(PyContextToken *self)
+{
+    Py_CLEAR(self->tok_ctx);
+    Py_CLEAR(self->tok_var);
+    Py_CLEAR(self->tok_oldval);
+    return 0;
+}
+
+static int
+token_tp_traverse(PyContextToken *self, visitproc visit, void *arg)
+{
+    Py_VISIT(self->tok_ctx);
+    Py_VISIT(self->tok_var);
+    Py_VISIT(self->tok_oldval);
+    return 0;
+}
+
+static void
+token_tp_dealloc(PyContextToken *self)
+{
+    PyObject_GC_UnTrack(self);
+    (void)token_tp_clear(self);
+    Py_TYPE(self)->tp_free(self);
+}
+
+static PyObject *
+token_tp_repr(PyContextToken *self)
+{
+    _PyUnicodeWriter writer;
+
+    _PyUnicodeWriter_Init(&writer);
+
+    if (_PyUnicodeWriter_WriteASCIIString(&writer, "<Token", 6) < 0) {
+        goto error;
+    }
+
+    if (self->tok_used) {
+        if (_PyUnicodeWriter_WriteASCIIString(&writer, " used", 5) < 0) {
+            goto error;
+        }
+    }
+
+    if (_PyUnicodeWriter_WriteASCIIString(&writer, " var=", 5) < 0) {
+        goto error;
+    }
+
+    PyObject *var = PyObject_Repr((PyObject *)self->tok_var);
+    if (var == NULL) {
+        goto error;
+    }
+    if (_PyUnicodeWriter_WriteStr(&writer, var) < 0) {
+        Py_DECREF(var);
+        goto error;
+    }
+    Py_DECREF(var);
+
+    PyObject *addr = PyUnicode_FromFormat(" at %p>", self);
+    if (addr == NULL) {
+        goto error;
+    }
+    if (_PyUnicodeWriter_WriteStr(&writer, addr) < 0) {
+        Py_DECREF(addr);
+        goto error;
+    }
+    Py_DECREF(addr);
+
+    return _PyUnicodeWriter_Finish(&writer);
+
+error:
+    _PyUnicodeWriter_Dealloc(&writer);
+    return NULL;
+}
+
+static PyObject *
+token_get_var(PyContextToken *self)
+{
+    Py_INCREF(self->tok_var);
+    return (PyObject *)self->tok_var;
+}
+
+static PyObject *
+token_get_old_value(PyContextToken *self)
+{
+    if (self->tok_oldval == NULL) {
+        return get_token_missing();
+    }
+
+    Py_INCREF(self->tok_oldval);
+    return self->tok_oldval;
+}
+
+static PyGetSetDef PyContextTokenType_getsetlist[] = {
+    {"var", (getter)token_get_var, NULL, NULL},
+    {"old_value", (getter)token_get_old_value, NULL, NULL},
+    {NULL}
+};
+
+PyTypeObject PyContextToken_Type = {
+    PyVarObject_HEAD_INIT(&PyType_Type, 0)
+    "Token",
+    sizeof(PyContextToken),
+    .tp_getset = PyContextTokenType_getsetlist,
+    .tp_dealloc = (destructor)token_tp_dealloc,
+    .tp_getattro = PyObject_GenericGetAttr,
+    .tp_flags = Py_TPFLAGS_DEFAULT | Py_TPFLAGS_HAVE_GC,
+    .tp_traverse = (traverseproc)token_tp_traverse,
+    .tp_clear = (inquiry)token_tp_clear,
+    .tp_new = token_tp_new,
+    .tp_free = PyObject_GC_Del,
+    .tp_hash = PyObject_HashNotImplemented,
+    .tp_repr = (reprfunc)token_tp_repr,
+};
+
+static PyContextToken *
+token_new(PyContext *ctx, PyContextVar *var, PyObject *val)
+{
+    PyContextToken *tok = PyObject_GC_New(PyContextToken, &PyContextToken_Type);
+    if (tok == NULL) {
+        return NULL;
+    }
+
+    Py_INCREF(ctx);
+    tok->tok_ctx = ctx;
+
+    Py_INCREF(var);
+    tok->tok_var = var;
+
+    Py_XINCREF(val);
+    tok->tok_oldval = val;
+
+    tok->tok_used = 0;
+
+    PyObject_GC_Track(tok);
+    return tok;
+}
+
+
+/////////////////////////// Token.MISSING
+
+
+static PyObject *_token_missing;
+
+
+typedef struct {
+    PyObject_HEAD
+} PyContextTokenMissing;
+
+
+static PyObject *
+context_token_missing_tp_repr(PyObject *self)
+{
+    return PyUnicode_FromString("<Token.MISSING>");
+}
+
+
+PyTypeObject PyContextTokenMissing_Type = {
+    PyVarObject_HEAD_INIT(&PyType_Type, 0)
+    "Token.MISSING",
+    sizeof(PyContextTokenMissing),
+    .tp_getattro = PyObject_GenericGetAttr,
+    .tp_flags = Py_TPFLAGS_DEFAULT,
+    .tp_repr = context_token_missing_tp_repr,
+};
+
+
+static PyObject *
+get_token_missing(void)
+{
+    if (_token_missing != NULL) {
+        Py_INCREF(_token_missing);
+        return _token_missing;
+    }
+
+    _token_missing = (PyObject *)PyObject_New(
+        PyContextTokenMissing, &PyContextTokenMissing_Type);
+    if (_token_missing == NULL) {
+        return NULL;
+    }
+
+    Py_INCREF(_token_missing);
+    return _token_missing;
+}
+
+
+///////////////////////////
+
+
+int
+PyContext_ClearFreeList(void)
+{
+    int size = ctx_freelist_len;
+    while (ctx_freelist_len) {
+        PyContext *ctx = ctx_freelist;
+        ctx_freelist = (PyContext *)ctx->ctx_weakreflist;
+        ctx->ctx_weakreflist = NULL;
+        PyObject_GC_Del(ctx);
+        ctx_freelist_len--;
+    }
+    return size;
+}
+
+
+void
+_PyContext_Fini(void)
+{
+    Py_CLEAR(_token_missing);
+    (void)PyContext_ClearFreeList();
+    (void)_PyHamt_Fini();
+}
+
+
+int
+_PyContext_Init(void)
+{
+    if (!_PyHamt_Init()) {
+        return 0;
+    }
+
+    if ((PyType_Ready(&PyContext_Type) < 0) ||
+        (PyType_Ready(&PyContextVar_Type) < 0) ||
+        (PyType_Ready(&PyContextToken_Type) < 0) ||
+        (PyType_Ready(&PyContextTokenMissing_Type) < 0))
+    {
+        return 0;
+    }
+
+    PyObject *missing = get_token_missing();
+    if (PyDict_SetItemString(
+        PyContextToken_Type.tp_dict, "MISSING", missing))
+    {
+        Py_DECREF(missing);
+        return 0;
+    }
+    Py_DECREF(missing);
+
+    return 1;
+}
diff --git a/Python/hamt.c b/Python/hamt.c
new file mode 100644
index 0000000000..8ba5082026
--- /dev/null
+++ b/Python/hamt.c
@@ -0,0 +1,2982 @@
+#include "Python.h"
+
+#include "structmember.h"
+#include "internal/pystate.h"
+#include "internal/hamt.h"
+
+/* popcnt support in Visual Studio */
+#ifdef _MSC_VER
+#include <intrin.h>
+#endif
+
+/*
+This file provides an implemention of an immutable mapping using the
+Hash Array Mapped Trie (or HAMT) datastructure.
+
+This design allows to have:
+
+1. Efficient copy: immutable mappings can be copied by reference,
+   making it an O(1) operation.
+
+2. Efficient mutations: due to structural sharing, only a portion of
+   the trie needs to be copied when the collection is mutated.  The
+   cost of set/delete operations is O(log N).
+
+3. Efficient lookups: O(log N).
+
+(where N is number of key/value items in the immutable mapping.)
+
+
+HAMT
+====
+
+The core idea of HAMT is that the shape of the trie is encoded into the
+hashes of keys.
+
+Say we want to store a K/V pair in our mapping.  First, we calculate the
+hash of K, let's say it's 19830128, or in binary:
+
+    0b1001011101001010101110000 = 19830128
+
+Now let's partition this bit representation of the hash into blocks of
+5 bits each:
+
+    0b00_00000_10010_11101_00101_01011_10000 = 19830128
+          (6)   (5)   (4)   (3)   (2)   (1)
+
+Each block of 5 bits represents a number betwen 0 and 31.  So if we have
+a tree that consists of nodes, each of which is an array of 32 pointers,
+those 5-bit blocks will encode a position on a single tree level.
+
+For example, storing the key K with hash 19830128, results in the following
+tree structure:
+
+                     (array of 32 pointers)
+                     +---+ -- +----+----+----+ -- +----+
+  root node          | 0 | .. | 15 | 16 | 17 | .. | 31 |   0b10000 = 16 (1)
+  (level 1)          +---+ -- +----+----+----+ -- +----+
+                                      |
+                     +---+ -- +----+----+----+ -- +----+
+  a 2nd level node   | 0 | .. | 10 | 11 | 12 | .. | 31 |   0b01011 = 11 (2)
+                     +---+ -- +----+----+----+ -- +----+
+                                      |
+                     +---+ -- +----+----+----+ -- +----+
+  a 3rd level node   | 0 | .. | 04 | 05 | 06 | .. | 31 |   0b01011 = 5  (3)
+                     +---+ -- +----+----+----+ -- +----+
+                                      |
+                     +---+ -- +----+----+----+----+
+  a 4th level node   | 0 | .. | 04 | 29 | 30 | 31 |        0b11101 = 29 (4)
+                     +---+ -- +----+----+----+----+
+                                      |
+                     +---+ -- +----+----+----+ -- +----+
+  a 5th level node   | 0 | .. | 17 | 18 | 19 | .. | 31 |   0b10010 = 18 (5)
+                     +---+ -- +----+----+----+ -- +----+
+                                      |
+                       +--------------+
+                       |
+                     +---+ -- +----+----+----+ -- +----+
+  a 6th level node   | 0 | .. | 15 | 16 | 17 | .. | 31 |   0b00000 = 0  (6)
+                     +---+ -- +----+----+----+ -- +----+
+                       |
+                       V -- our value (or collision)
+
+To rehash: for a K/V pair, the hash of K encodes where in the tree V will
+be stored.
+
+To optimize memory footprint and handle hash collisions, our implementation
+uses three different types of nodes:
+
+ * A Bitmap node;
+ * An Array node;
+ * A Collision node.
+
+Because we implement an immutable dictionary, our nodes are also
+immutable.  Therefore, when we need to modify a node, we copy it, and
+do that modification to the copy.
+
+
+Array Nodes
+-----------
+
+These nodes are very simple.  Essentially they are arrays of 32 pointers
+we used to illustrate the high-level idea in the previous section.
+
+We use Array nodes only when we need to store more than 16 pointers
+in a single node.
+
+Array nodes do not store key objects or value objects.  They are used
+only as an indirection level - their pointers point to other nodes in
+the tree.
+
+
+Bitmap Node
+-----------
+
+Allocating a new 32-pointers array for every node of our tree would be
+very expensive.  Unless we store millions of keys, most of tree nodes would
+be very sparse.
+
+When we have less than 16 elements in a node, we don't want to use the
+Array node, that would mean that we waste a lot of memory.  Instead,
+we can use bitmap compression and can have just as many pointers
+as we need!
+
+Bitmap nodes consist of two fields:
+
+1. An array of pointers.  If a Bitmap node holds N elements, the
+   array will be of N pointers.
+
+2. A 32bit integer -- a bitmap field.  If an N-th bit is set in the
+   bitmap, it means that the node has an N-th element.
+
+For example, say we need to store a 3 elements sparse array:
+
+   +---+  --  +---+  --  +----+  --  +----+
+   | 0 |  ..  | 4 |  ..  | 11 |  ..  | 17 |
+   +---+  --  +---+  --  +----+  --  +----+
+                |          |           |
+                o1         o2          o3
+
+We allocate a three-pointer Bitmap node.  Its bitmap field will be
+then set to:
+
+   0b_00100_00010_00000_10000 == (1 << 17) | (1 << 11) | (1 << 4)
+
+To check if our Bitmap node has an I-th element we can do:
+
+   bitmap & (1 << I)
+
+
+And here's a formula to calculate a position in our pointer array
+which would correspond to an I-th element:
+
+   popcount(bitmap & ((1 << I) - 1))
+
+
+Let's break it down:
+
+ * `popcount` is a function that returns a number of bits set to 1;
+
+ * `((1 << I) - 1)` is a mask to filter the bitmask to contain bits
+   set to the *right* of our bit.
+
+
+So for our 17, 11, and 4 indexes:
+
+ * bitmap & ((1 << 17) - 1) == 0b100000010000 => 2 bits are set => index is 2.
+
+ * bitmap & ((1 << 11) - 1) == 0b10000 => 1 bit is set => index is 1.
+
+ * bitmap & ((1 << 4) - 1) == 0b0 => 0 bits are set => index is 0.
+
+
+To conclude: Bitmap nodes are just like Array nodes -- they can store
+a number of pointers, but use bitmap compression to eliminate unused
+pointers.
+
+
+Bitmap nodes have two pointers for each item:
+
+  +----+----+----+----+  --  +----+----+
+  | k1 | v1 | k2 | v2 |  ..  | kN | vN |
+  +----+----+----+----+  --  +----+----+
+
+When kI == NULL, vI points to another tree level.
+
+When kI != NULL, the actual key object is stored in kI, and its
+value is stored in vI.
+
+
+Collision Nodes
+---------------
+
+Collision nodes are simple arrays of pointers -- two pointers per
+key/value.  When there's a hash collision, say for k1/v1 and k2/v2
+we have `hash(k1)==hash(k2)`.  Then our collision node will be:
+
+  +----+----+----+----+
+  | k1 | v1 | k2 | v2 |
+  +----+----+----+----+
+
+
+Tree Structure
+--------------
+
+All nodes are PyObjects.
+
+The `PyHamtObject` object has a pointer to the root node (h_root),
+and has a length field (h_count).
+
+High-level functions accept a PyHamtObject object and dispatch to
+lower-level functions depending on what kind of node h_root points to.
+
+
+Operations
+==========
+
+There are three fundamental operations on an immutable dictionary:
+
+1. "o.assoc(k, v)" will return a new immutable dictionary, that will be
+   a copy of "o", but with the "k/v" item set.
+
+   Functions in this file:
+
+        hamt_node_assoc, hamt_node_bitmap_assoc,
+        hamt_node_array_assoc, hamt_node_collision_assoc
+
+   `hamt_node_assoc` function accepts a node object, and calls
+   other functions depending on its actual type.
+
+2. "o.find(k)" will lookup key "k" in "o".
+
+   Functions:
+
+        hamt_node_find, hamt_node_bitmap_find,
+        hamt_node_array_find, hamt_node_collision_find
+
+3. "o.without(k)" will return a new immutable dictionary, that will be
+   a copy of "o", buth without the "k" key.
+
+   Functions:
+
+        hamt_node_without, hamt_node_bitmap_without,
+        hamt_node_array_without, hamt_node_collision_without
+
+
+Further Reading
+===============
+
+1. http://blog.higher-order.net/2009/09/08/understanding-clojures-persistenthashmap-deftwice.html
+
+2. http://blog.higher-order.net/2010/08/16/assoc-and-clojures-persistenthashmap-part-ii.html
+
+3. Clojure's PersistentHashMap implementation:
+   https://github.com/clojure/clojure/blob/master/src/jvm/clojure/lang/PersistentHashMap.java
+
+
+Debug
+=====
+
+The HAMT datatype is accessible for testing purposes under the
+`_testcapi` module:
+
+    >>> from _testcapi import hamt
+    >>> h = hamt()
+    >>> h2 = h.set('a', 2)
+    >>> h3 = h2.set('b', 3)
+    >>> list(h3)
+    ['a', 'b']
+
+When CPython is built in debug mode, a '__dump__()' method is available
+to introspect the tree:
+
+    >>> print(h3.__dump__())
+    HAMT(len=2):
+        BitmapNode(size=4 count=2 bitmap=0b110 id=0x10eb9d9e8):
+            'a': 2
+            'b': 3
+*/
+
+
+#define IS_ARRAY_NODE(node)     (Py_TYPE(node) == &_PyHamt_ArrayNode_Type)
+#define IS_BITMAP_NODE(node)    (Py_TYPE(node) == &_PyHamt_BitmapNode_Type)
+#define IS_COLLISION_NODE(node) (Py_TYPE(node) == &_PyHamt_CollisionNode_Type)
+
+
+/* Return type for 'find' (lookup a key) functions.
+
+   * F_ERROR - an error occurred;
+   * F_NOT_FOUND - the key was not found;
+   * F_FOUND - the key was found.
+*/
+typedef enum {F_ERROR, F_NOT_FOUND, F_FOUND} hamt_find_t;
+
+
+/* Return type for 'without' (delete a key) functions.
+
+   * W_ERROR - an error occurred;
+   * W_NOT_FOUND - the key was not found: there's nothing to delete;
+   * W_EMPTY - the key was found: the node/tree would be empty
+     if the key is deleted;
+   * W_NEWNODE - the key was found: a new node/tree is returned
+     without that key.
+*/
+typedef enum {W_ERROR, W_NOT_FOUND, W_EMPTY, W_NEWNODE} hamt_without_t;
+
+
+/* Low-level iterator protocol type.
+
+   * I_ITEM - a new item has been yielded;
+   * I_END - the whole tree was visited (similar to StopIteration).
+*/
+typedef enum {I_ITEM, I_END} hamt_iter_t;
+
+
+#define HAMT_ARRAY_NODE_SIZE 32
+
+
+typedef struct {
+    PyObject_HEAD
+    PyHamtNode *a_array[HAMT_ARRAY_NODE_SIZE];
+    Py_ssize_t a_count;
+} PyHamtNode_Array;
+
+
+typedef struct {
+    PyObject_VAR_HEAD
+    uint32_t b_bitmap;
+    PyObject *b_array[1];
+} PyHamtNode_Bitmap;
+
+
+typedef struct {
+    PyObject_VAR_HEAD
+    int32_t c_hash;
+    PyObject *c_array[1];
+} PyHamtNode_Collision;
+
+
+static PyHamtNode_Bitmap *_empty_bitmap_node;
+static PyHamtObject *_empty_hamt;
+
+
+static PyHamtObject *
+hamt_alloc(void);
+
+static PyHamtNode *
+hamt_node_assoc(PyHamtNode *node,
+                uint32_t shift, int32_t hash,
+                PyObject *key, PyObject *val, int* added_leaf);
+
+static hamt_without_t
+hamt_node_without(PyHamtNode *node,
+                  uint32_t shift, int32_t hash,
+                  PyObject *key,
+                  PyHamtNode **new_node);
+
+static hamt_find_t
+hamt_node_find(PyHamtNode *node,
+               uint32_t shift, int32_t hash,
+               PyObject *key, PyObject **val);
+
+#ifdef Py_DEBUG
+static int
+hamt_node_dump(PyHamtNode *node,
+               _PyUnicodeWriter *writer, int level);
+#endif
+
+static PyHamtNode *
+hamt_node_array_new(Py_ssize_t);
+
+static PyHamtNode *
+hamt_node_collision_new(int32_t hash, Py_ssize_t size);
+
+static inline Py_ssize_t
+hamt_node_collision_count(PyHamtNode_Collision *node);
+
+
+#ifdef Py_DEBUG
+static void
+_hamt_node_array_validate(void *o)
+{
+    assert(IS_ARRAY_NODE(o));
+    PyHamtNode_Array *node = (PyHamtNode_Array*)(o);
+    Py_ssize_t i = 0, count = 0;
+    for (; i < HAMT_ARRAY_NODE_SIZE; i++) {
+        if (node->a_array[i] != NULL) {
+            count++;
+        }
+    }
+    assert(count == node->a_count);
+}
+
+#define VALIDATE_ARRAY_NODE(NODE) \
+    do { _hamt_node_array_validate(NODE); } while (0);
+#else
+#define VALIDATE_ARRAY_NODE(NODE)
+#endif
+
+
+/* Returns -1 on error */
+static inline int32_t
+hamt_hash(PyObject *o)
+{
+    Py_hash_t hash = PyObject_Hash(o);
+
+#if SIZEOF_PY_HASH_T <= 4
+    return hash;
+#else
+    if (hash == -1) {
+        /* exception */
+        return -1;
+    }
+
+    /* While it's suboptimal to reduce Python's 64 bit hash to
+       32 bits via XOR, it seems that the resulting hash function
+       is good enough (this is also how Long type is hashed in Java.)
+       Storing 10, 100, 1000 Python strings results in a relatively
+       shallow and uniform tree structure.
+
+       Please don't change this hashing algorithm, as there are many
+       tests that test some exact tree shape to cover all code paths.
+    */
+    int32_t xored = (int32_t)(hash & 0xffffffffl) ^ (int32_t)(hash >> 32);
+    return xored == -1 ? -2 : xored;
+#endif
+}
+
+static inline uint32_t
+hamt_mask(int32_t hash, uint32_t shift)
+{
+    return (((uint32_t)hash >> shift) & 0x01f);
+}
+
+static inline uint32_t
+hamt_bitpos(int32_t hash, uint32_t shift)
+{
+    return (uint32_t)1 << hamt_mask(hash, shift);
+}
+
+static inline uint32_t
+hamt_bitcount(uint32_t i)
+{
+#if defined(__GNUC__) && (__GNUC__ > 4)
+    return (uint32_t)__builtin_popcountl(i);
+#elif defined(__clang__) && (__clang_major__ > 3)
+    return (uint32_t)__builtin_popcountl(i);
+#elif defined(_MSC_VER)
+    return (uint32_t)__popcnt(i);
+#else
+    /* https://graphics.stanford.edu/~seander/bithacks.html */
+    i = i - ((i >> 1) & 0x55555555);
+    i = (i & 0x33333333) + ((i >> 2) & 0x33333333);
+    return ((i + (i >> 4) & 0xF0F0F0F) * 0x1010101) >> 24;
+#endif
+}
+
+static inline uint32_t
+hamt_bitindex(uint32_t bitmap, uint32_t bit)
+{
+    return hamt_bitcount(bitmap & (bit - 1));
+}
+
+
+/////////////////////////////////// Dump Helpers
+#ifdef Py_DEBUG
+
+static int
+_hamt_dump_ident(_PyUnicodeWriter *writer, int level)
+{
+    /* Write `'    ' * level` to the `writer` */
+    PyObject *str = NULL;
+    PyObject *num = NULL;
+    PyObject *res = NULL;
+    int ret = -1;
+
+    str = PyUnicode_FromString("    ");
+    if (str == NULL) {
+        goto error;
+    }
+
+    num = PyLong_FromLong((long)level);
+    if (num == NULL) {
+        goto error;
+    }
+
+    res = PyNumber_Multiply(str, num);
+    if (res == NULL) {
+        goto error;
+    }
+
+    ret = _PyUnicodeWriter_WriteStr(writer, res);
+
+error:
+    Py_XDECREF(res);
+    Py_XDECREF(str);
+    Py_XDECREF(num);
+    return ret;
+}
+
+static int
+_hamt_dump_format(_PyUnicodeWriter *writer, const char *format, ...)
+{
+    /* A convenient helper combining _PyUnicodeWriter_WriteStr and
+       PyUnicode_FromFormatV.
+    */
+    PyObject* msg;
+    int ret;
+
+    va_list vargs;
+#ifdef HAVE_STDARG_PROTOTYPES
+    va_start(vargs, format);
+#else
+    va_start(vargs);
+#endif
+    msg = PyUnicode_FromFormatV(format, vargs);
+    va_end(vargs);
+
+    if (msg == NULL) {
+        return -1;
+    }
+
+    ret = _PyUnicodeWriter_WriteStr(writer, msg);
+    Py_DECREF(msg);
+    return ret;
+}
+
+#endif  /* Py_DEBUG */
+/////////////////////////////////// Bitmap Node
+
+
+static PyHamtNode *
+hamt_node_bitmap_new(Py_ssize_t size)
+{
+    /* Create a new bitmap node of size 'size' */
+
+    PyHamtNode_Bitmap *node;
+    Py_ssize_t i;
+
+    assert(size >= 0);
+    assert(size % 2 == 0);
+
+    if (size == 0 && _empty_bitmap_node != NULL) {
+        Py_INCREF(_empty_bitmap_node);
+        return (PyHamtNode *)_empty_bitmap_node;
+    }
+
+    /* No freelist; allocate a new bitmap node */
+    node = PyObject_GC_NewVar(
+        PyHamtNode_Bitmap, &_PyHamt_BitmapNode_Type, size);
+    if (node == NULL) {
+        return NULL;
+    }
+
+    Py_SIZE(node) = size;
+
+    for (i = 0; i < size; i++) {
+        node->b_array[i] = NULL;
+    }
+
+    node->b_bitmap = 0;
+
+    _PyObject_GC_TRACK(node);
+
+    if (size == 0 && _empty_bitmap_node == NULL) {
+        /* Since bitmap nodes are immutable, we can cache the instance
+           for size=0 and reuse it whenever we need an empty bitmap node.
+        */
+        _empty_bitmap_node = node;
+        Py_INCREF(_empty_bitmap_node);
+    }
+
+    return (PyHamtNode *)node;
+}
+
+static inline Py_ssize_t
+hamt_node_bitmap_count(PyHamtNode_Bitmap *node)
+{
+    return Py_SIZE(node) / 2;
+}
+
+static PyHamtNode_Bitmap *
+hamt_node_bitmap_clone(PyHamtNode_Bitmap *node)
+{
+    /* Clone a bitmap node; return a new one with the same child notes. */
+
+    PyHamtNode_Bitmap *clone;
+    Py_ssize_t i;
+
+    clone = (PyHamtNode_Bitmap *)hamt_node_bitmap_new(Py_SIZE(node));
+    if (clone == NULL) {
+        return NULL;
+    }
+
+    for (i = 0; i < Py_SIZE(node); i++) {
+        Py_XINCREF(node->b_array[i]);
+        clone->b_array[i] = node->b_array[i];
+    }
+
+    clone->b_bitmap = node->b_bitmap;
+    return clone;
+}
+
+static PyHamtNode_Bitmap *
+hamt_node_bitmap_clone_without(PyHamtNode_Bitmap *o, uint32_t bit)
+{
+    assert(bit & o->b_bitmap);
+    assert(hamt_node_bitmap_count(o) > 1);
+
+    PyHamtNode_Bitmap *new = (PyHamtNode_Bitmap *)hamt_node_bitmap_new(
+        Py_SIZE(o) - 2);
+    if (new == NULL) {
+        return NULL;
+    }
+
+    uint32_t idx = hamt_bitindex(o->b_bitmap, bit);
+    uint32_t key_idx = 2 * idx;
+    uint32_t val_idx = key_idx + 1;
+    uint32_t i;
+
+    for (i = 0; i < key_idx; i++) {
+        Py_XINCREF(o->b_array[i]);
+        new->b_array[i] = o->b_array[i];
+    }
+
+    for (i = val_idx + 1; i < Py_SIZE(o); i++) {
+        Py_XINCREF(o->b_array[i]);
+        new->b_array[i - 2] = o->b_array[i];
+    }
+
+    new->b_bitmap = o->b_bitmap & ~bit;
+    return new;
+}
+
+static PyHamtNode *
+hamt_node_new_bitmap_or_collision(uint32_t shift,
+                                  PyObject *key1, PyObject *val1,
+                                  int32_t key2_hash,
+                                  PyObject *key2, PyObject *val2)
+{
+    /* Helper method.  Creates a new node for key1/val and key2/val2
+       pairs.
+
+       If key1 hash is equal to the hash of key2, a Collision node
+       will be created.  If they are not equal, a Bitmap node is
+       created.
+    */
+
+    int32_t key1_hash = hamt_hash(key1);
+    if (key1_hash == -1) {
+        return NULL;
+    }
+
+    if (key1_hash == key2_hash) {
+        PyHamtNode_Collision *n;
+        n = (PyHamtNode_Collision *)hamt_node_collision_new(key1_hash, 4);
+        if (n == NULL) {
+            return NULL;
+        }
+
+        Py_INCREF(key1);
+        n->c_array[0] = key1;
+        Py_INCREF(val1);
+        n->c_array[1] = val1;
+
+        Py_INCREF(key2);
+        n->c_array[2] = key2;
+        Py_INCREF(val2);
+        n->c_array[3] = val2;
+
+        return (PyHamtNode *)n;
+    }
+    else {
+        int added_leaf = 0;
+        PyHamtNode *n = hamt_node_bitmap_new(0);
+        if (n == NULL) {
+            return NULL;
+        }
+
+        PyHamtNode *n2 = hamt_node_assoc(
+            n, shift, key1_hash, key1, val1, &added_leaf);
+        Py_DECREF(n);
+        if (n2 == NULL) {
+            return NULL;
+        }
+
+        n = hamt_node_assoc(n2, shift, key2_hash, key2, val2, &added_leaf);
+        Py_DECREF(n2);
+        if (n == NULL) {
+            return NULL;
+        }
+
+        return n;
+    }
+}
+
+static PyHamtNode *
+hamt_node_bitmap_assoc(PyHamtNode_Bitmap *self,
+                       uint32_t shift, int32_t hash,
+                       PyObject *key, PyObject *val, int* added_leaf)
+{
+    /* assoc operation for bitmap nodes.
+
+       Return: a new node, or self if key/val already is in the
+       collection.
+
+       'added_leaf' is later used in '_PyHamt_Assoc' to determine if
+       `hamt.set(key, val)` increased the size of the collection.
+    */
+
+    uint32_t bit = hamt_bitpos(hash, shift);
+    uint32_t idx = hamt_bitindex(self->b_bitmap, bit);
+
+    /* Bitmap node layout:
+
+    +------+------+------+------+  ---  +------+------+
+    | key1 | val1 | key2 | val2 |  ...  | keyN | valN |
+    +------+------+------+------+  ---  +------+------+
+    where `N < Py_SIZE(node)`.
+
+    The `node->b_bitmap` field is a bitmap.  For a given
+    `(shift, hash)` pair we can determine:
+
+     - If this node has the corresponding key/val slots.
+     - The index of key/val slots.
+    */
+
+    if (self->b_bitmap & bit) {
+        /* The key is set in this node */
+
+        uint32_t key_idx = 2 * idx;
+        uint32_t val_idx = key_idx + 1;
+
+        assert(val_idx < Py_SIZE(self));
+
+        PyObject *key_or_null = self->b_array[key_idx];
+        PyObject *val_or_node = self->b_array[val_idx];
+
+        if (key_or_null == NULL) {
+            /* key is NULL.  This means that we have a few keys
+               that have the same (hash, shift) pair. */
+
+            assert(val_or_node != NULL);
+
+            PyHamtNode *sub_node = hamt_node_assoc(
+                (PyHamtNode *)val_or_node,
+                shift + 5, hash, key, val, added_leaf);
+            if (sub_node == NULL) {
+                return NULL;
+            }
+
+            if (val_or_node == (PyObject *)sub_node) {
+                Py_DECREF(sub_node);
+                Py_INCREF(self);
+                return (PyHamtNode *)self;
+            }
+
+            PyHamtNode_Bitmap *ret = hamt_node_bitmap_clone(self);
+            if (ret == NULL) {
+                return NULL;
+            }
+            Py_SETREF(ret->b_array[val_idx], (PyObject*)sub_node);
+            return (PyHamtNode *)ret;
+        }
+
+        assert(key != NULL);
+        /* key is not NULL.  This means that we have only one other
+           key in this collection that matches our hash for this shift. */
+
+        int comp_err = PyObject_RichCompareBool(key, key_or_null, Py_EQ);
+        if (comp_err < 0) {  /* exception in __eq__ */
+            return NULL;
+        }
+        if (comp_err == 1) {  /* key == key_or_null */
+            if (val == val_or_node) {
+                /* we already have the same key/val pair; return self. */
+                Py_INCREF(self);
+                return (PyHamtNode *)self;
+            }
+
+            /* We're setting a new value for the key we had before.
+               Make a new bitmap node with a replaced value, and return it. */
+            PyHamtNode_Bitmap *ret = hamt_node_bitmap_clone(self);
+            if (ret == NULL) {
+                return NULL;
+            }
+            Py_INCREF(val);
+            Py_SETREF(ret->b_array[val_idx], val);
+            return (PyHamtNode *)ret;
+        }
+
+        /* It's a new key, and it has the same index as *one* another key.
+           We have a collision.  We need to create a new node which will
+           combine the existing key and the key we're adding.
+
+           `hamt_node_new_bitmap_or_collision` will either create a new
+           Collision node if the keys have identical hashes, or
+           a new Bitmap node.
+        */
+        PyHamtNode *sub_node = hamt_node_new_bitmap_or_collision(
+            shift + 5,
+            key_or_null, val_or_node,  /* existing key/val */
+            hash,
+            key, val  /* new key/val */
+        );
+        if (sub_node == NULL) {
+            return NULL;
+        }
+
+        PyHamtNode_Bitmap *ret = hamt_node_bitmap_clone(self);
+        if (ret == NULL) {
+            Py_DECREF(sub_node);
+            return NULL;
+        }
+        Py_SETREF(ret->b_array[key_idx], NULL);
+        Py_SETREF(ret->b_array[val_idx], (PyObject *)sub_node);
+
+        *added_leaf = 1;
+        return (PyHamtNode *)ret;
+    }
+    else {
+        /* There was no key before with the same (shift,hash). */
+
+        uint32_t n = hamt_bitcount(self->b_bitmap);
+
+        if (n >= 16) {
+            /* When we have a situation where we want to store more
+               than 16 nodes at one level of the tree, we no longer
+               want to use the Bitmap node with bitmap encoding.
+
+               Instead we start using an Array node, which has
+               simpler (faster) implementation at the expense of
+               having prealocated 32 pointers for its keys/values
+               pairs.
+
+               Small hamt objects (<30 keys) usually don't have any
+               Array nodes at all.  Betwen ~30 and ~400 keys hamt
+               objects usually have one Array node, and usually it's
+               a root node.
+            */
+
+            uint32_t jdx = hamt_mask(hash, shift);
+            /* 'jdx' is the index of where the new key should be added
+               in the new Array node we're about to create. */
+
+            PyHamtNode *empty = NULL;
+            PyHamtNode_Array *new_node = NULL;
+            PyHamtNode *res = NULL;
+
+            /* Create a new Array node. */
+            new_node = (PyHamtNode_Array *)hamt_node_array_new(n + 1);
+            if (new_node == NULL) {
+                goto fin;
+            }
+
+            /* Create an empty bitmap node for the next
+               hamt_node_assoc call. */
+            empty = hamt_node_bitmap_new(0);
+            if (empty == NULL) {
+                goto fin;
+            }
+
+            /* Make a new bitmap node for the key/val we're adding.
+               Set that bitmap node to new-array-node[jdx]. */
+            new_node->a_array[jdx] = hamt_node_assoc(
+                empty, shift + 5, hash, key, val, added_leaf);
+            if (new_node->a_array[jdx] == NULL) {
+                goto fin;
+            }
+
+            /* Copy existing key/value pairs from the current Bitmap
+               node to the new Array node we've just created. */
+            Py_ssize_t i, j;
+            for (i = 0, j = 0; i < HAMT_ARRAY_NODE_SIZE; i++) {
+                if (((self->b_bitmap >> i) & 1) != 0) {
+                    /* Ensure we don't accidentally override `jdx` element
+                       we set few lines above.
+                    */
+                    assert(new_node->a_array[i] == NULL);
+
+                    if (self->b_array[j] == NULL) {
+                        new_node->a_array[i] =
+                            (PyHamtNode *)self->b_array[j + 1];
+                        Py_INCREF(new_node->a_array[i]);
+                    }
+                    else {
+                        int32_t rehash = hamt_hash(self->b_array[j]);
+                        if (rehash == -1) {
+                            goto fin;
+                        }
+
+                        new_node->a_array[i] = hamt_node_assoc(
+                            empty, shift + 5,
+                            rehash,
+                            self->b_array[j],
+                            self->b_array[j + 1],
+                            added_leaf);
+
+                        if (new_node->a_array[i] == NULL) {
+                            goto fin;
+                        }
+                    }
+                    j += 2;
+                }
+            }
+
+            VALIDATE_ARRAY_NODE(new_node)
+
+            /* That's it! */
+            res = (PyHamtNode *)new_node;
+
+        fin:
+            Py_XDECREF(empty);
+            if (res == NULL) {
+                Py_XDECREF(new_node);
+            }
+            return res;
+        }
+        else {
+            /* We have less than 16 keys at this level; let's just
+               create a new bitmap node out of this node with the
+               new key/val pair added. */
+
+            uint32_t key_idx = 2 * idx;
+            uint32_t val_idx = key_idx + 1;
+            Py_ssize_t i;
+
+            *added_leaf = 1;
+
+            /* Allocate new Bitmap node which can have one more key/val
+               pair in addition to what we have already. */
+            PyHamtNode_Bitmap *new_node =
+                (PyHamtNode_Bitmap *)hamt_node_bitmap_new(2 * (n + 1));
+            if (new_node == NULL) {
+                return NULL;
+            }
+
+            /* Copy all keys/values that will be before the new key/value
+               we are adding. */
+            for (i = 0; i < key_idx; i++) {
+                Py_XINCREF(self->b_array[i]);
+                new_node->b_array[i] = self->b_array[i];
+            }
+
+            /* Set the new key/value to the new Bitmap node. */
+            Py_INCREF(key);
+            new_node->b_array[key_idx] = key;
+            Py_INCREF(val);
+            new_node->b_array[val_idx] = val;
+
+            /* Copy all keys/values that will be after the new key/value
+               we are adding. */
+            for (i = key_idx; i < Py_SIZE(self); i++) {
+                Py_XINCREF(self->b_array[i]);
+                new_node->b_array[i + 2] = self->b_array[i];
+            }
+
+            new_node->b_bitmap = self->b_bitmap | bit;
+            return (PyHamtNode *)new_node;
+        }
+    }
+}
+
+static hamt_without_t
+hamt_node_bitmap_without(PyHamtNode_Bitmap *self,
+                         uint32_t shift, int32_t hash,
+                         PyObject *key,
+                         PyHamtNode **new_node)
+{
+    uint32_t bit = hamt_bitpos(hash, shift);
+    if ((self->b_bitmap & bit) == 0) {
+        return W_NOT_FOUND;
+    }
+
+    uint32_t idx = hamt_bitindex(self->b_bitmap, bit);
+
+    uint32_t key_idx = 2 * idx;
+    uint32_t val_idx = key_idx + 1;
+
+    PyObject *key_or_null = self->b_array[key_idx];
+    PyObject *val_or_node = self->b_array[val_idx];
+
+    if (key_or_null == NULL) {
+        /* key == NULL means that 'value' is another tree node. */
+
+        PyHamtNode *sub_node = NULL;
+
+        hamt_without_t res = hamt_node_without(
+            (PyHamtNode *)val_or_node,
+            shift + 5, hash, key, &sub_node);
+
+        switch (res) {
+            case W_EMPTY:
+                /* It's impossible for us to receive a W_EMPTY here:
+
+                    - Array nodes are converted to Bitmap nodes when
+                      we delete 16th item from them;
+
+                    - Collision nodes are converted to Bitmap when
+                      there is one item in them;
+
+                    - Bitmap node's without() inlines single-item
+                      sub-nodes.
+
+                   So in no situation we can have a single-item
+                   Bitmap child of another Bitmap node.
+                */
+                Py_UNREACHABLE();
+
+            case W_NEWNODE: {
+                assert(sub_node != NULL);
+
+                if (IS_BITMAP_NODE(sub_node)) {
+                    PyHamtNode_Bitmap *sub_tree = (PyHamtNode_Bitmap *)sub_node;
+                    if (hamt_node_bitmap_count(sub_tree) == 1 &&
+                            sub_tree->b_array[0] != NULL)
+                    {
+                        /* A bitmap node with one key/value pair.  Just
+                           merge it into this node.
+
+                           Note that we don't inline Bitmap nodes that
+                           have a NULL key -- those nodes point to another
+                           tree level, and we cannot simply move tree levels
+                           up or down.
+                        */
+
+                        PyHamtNode_Bitmap *clone = hamt_node_bitmap_clone(self);
+                        if (clone == NULL) {
+                            Py_DECREF(sub_node);
+                            return W_ERROR;
+                        }
+
+                        PyObject *key = sub_tree->b_array[0];
+                        PyObject *val = sub_tree->b_array[1];
+
+                        Py_INCREF(key);
+                        Py_XSETREF(clone->b_array[key_idx], key);
+                        Py_INCREF(val);
+                        Py_SETREF(clone->b_array[val_idx], val);
+
+                        Py_DECREF(sub_tree);
+
+                        *new_node = (PyHamtNode *)clone;
+                        return W_NEWNODE;
+                    }
+                }
+
+#ifdef Py_DEBUG
+                /* Ensure that Collision.without implementation
+                   converts to Bitmap nodes itself.
+                */
+                if (IS_COLLISION_NODE(sub_node)) {
+                    assert(hamt_node_collision_count(
+                            (PyHamtNode_Collision*)sub_node) > 1);
+                }
+#endif
+
+                PyHamtNode_Bitmap *clone = hamt_node_bitmap_clone(self);
+                Py_SETREF(clone->b_array[val_idx],
+                          (PyObject *)sub_node);  /* borrow */
+
+                *new_node = (PyHamtNode *)clone;
+                return W_NEWNODE;
+            }
+
+            case W_ERROR:
+            case W_NOT_FOUND:
+                assert(sub_node == NULL);
+                return res;
+
+            default:
+                Py_UNREACHABLE();
+        }
+    }
+    else {
+        /* We have a regular key/value pair */
+
+        int cmp = PyObject_RichCompareBool(key_or_null, key, Py_EQ);
+        if (cmp < 0) {
+            return W_ERROR;
+        }
+        if (cmp == 0) {
+            return W_NOT_FOUND;
+        }
+
+        if (hamt_node_bitmap_count(self) == 1) {
+            return W_EMPTY;
+        }
+
+        *new_node = (PyHamtNode *)
+            hamt_node_bitmap_clone_without(self, bit);
+        if (*new_node == NULL) {
+            return W_ERROR;
+        }
+
+        return W_NEWNODE;
+    }
+}
+
+static hamt_find_t
+hamt_node_bitmap_find(PyHamtNode_Bitmap *self,
+                      uint32_t shift, int32_t hash,
+                      PyObject *key, PyObject **val)
+{
+    /* Lookup a key in a Bitmap node. */
+
+    uint32_t bit = hamt_bitpos(hash, shift);
+    uint32_t idx;
+    uint32_t key_idx;
+    uint32_t val_idx;
+    PyObject *key_or_null;
+    PyObject *val_or_node;
+    int comp_err;
+
+    if ((self->b_bitmap & bit) == 0) {
+        return F_NOT_FOUND;
+    }
+
+    idx = hamt_bitindex(self->b_bitmap, bit);
+    assert(idx >= 0);
+    key_idx = idx * 2;
+    val_idx = key_idx + 1;
+
+    assert(val_idx < Py_SIZE(self));
+
+    key_or_null = self->b_array[key_idx];
+    val_or_node = self->b_array[val_idx];
+
+    if (key_or_null == NULL) {
+        /* There are a few keys that have the same hash at the current shift
+           that match our key.  Dispatch the lookup further down the tree. */
+        assert(val_or_node != NULL);
+        return hamt_node_find((PyHamtNode *)val_or_node,
+                              shift + 5, hash, key, val);
+    }
+
+    /* We have only one key -- a potential match.  Let's compare if the
+       key we are looking at is equal to the key we are looking for. */
+    assert(key != NULL);
+    comp_err = PyObject_RichCompareBool(key, key_or_null, Py_EQ);
+    if (comp_err < 0) {  /* exception in __eq__ */
+        return F_ERROR;
+    }
+    if (comp_err == 1) {  /* key == key_or_null */
+        *val = val_or_node;
+        return F_FOUND;
+    }
+
+    return F_NOT_FOUND;
+}
+
+static int
+hamt_node_bitmap_traverse(PyHamtNode_Bitmap *self, visitproc visit, void *arg)
+{
+    /* Bitmap's tp_traverse */
+
+    Py_ssize_t i;
+
+    for (i = Py_SIZE(self); --i >= 0; ) {
+        Py_VISIT(self->b_array[i]);
+    }
+
+    return 0;
+}
+
+static void
+hamt_node_bitmap_dealloc(PyHamtNode_Bitmap *self)
+{
+    /* Bitmap's tp_dealloc */
+
+    Py_ssize_t len = Py_SIZE(self);
+    Py_ssize_t i;
+
+    PyObject_GC_UnTrack(self);
+    Py_TRASHCAN_SAFE_BEGIN(self)
+
+    if (len > 0) {
+        i = len;
+        while (--i >= 0) {
+            Py_XDECREF(self->b_array[i]);
+        }
+    }
+
+    Py_TYPE(self)->tp_free((PyObject *)self);
+    Py_TRASHCAN_SAFE_END(self)
+}
+
+#ifdef Py_DEBUG
+static int
+hamt_node_bitmap_dump(PyHamtNode_Bitmap *node,
+                      _PyUnicodeWriter *writer, int level)
+{
+    /* Debug build: __dump__() method implementation for Bitmap nodes. */
+
+    Py_ssize_t i;
+    PyObject *tmp1;
+    PyObject *tmp2;
+
+    if (_hamt_dump_ident(writer, level + 1)) {
+        goto error;
+    }
+
+    if (_hamt_dump_format(writer, "BitmapNode(size=%zd count=%zd ",
+                          Py_SIZE(node), Py_SIZE(node) / 2))
+    {
+        goto error;
+    }
+
+    tmp1 = PyLong_FromUnsignedLong(node->b_bitmap);
+    if (tmp1 == NULL) {
+        goto error;
+    }
+    tmp2 = _PyLong_Format(tmp1, 2);
+    Py_DECREF(tmp1);
+    if (tmp2 == NULL) {
+        goto error;
+    }
+    if (_hamt_dump_format(writer, "bitmap=%S id=%p):\n", tmp2, node)) {
+        Py_DECREF(tmp2);
+        goto error;
+    }
+    Py_DECREF(tmp2);
+
+    for (i = 0; i < Py_SIZE(node); i += 2) {
+        PyObject *key_or_null = node->b_array[i];
+        PyObject *val_or_node = node->b_array[i + 1];
+
+        if (_hamt_dump_ident(writer, level + 2)) {
+            goto error;
+        }
+
+        if (key_or_null == NULL) {
+            if (_hamt_dump_format(writer, "NULL:\n")) {
+                goto error;
+            }
+
+            if (hamt_node_dump((PyHamtNode *)val_or_node,
+                               writer, level + 2))
+            {
+                goto error;
+            }
+        }
+        else {
+            if (_hamt_dump_format(writer, "%R: %R", key_or_null,
+                                  val_or_node))
+            {
+                goto error;
+            }
+        }
+
+        if (_hamt_dump_format(writer, "\n")) {
+            goto error;
+        }
+    }
+
+    return 0;
+error:
+    return -1;
+}
+#endif  /* Py_DEBUG */
+
+
+/////////////////////////////////// Collision Node
+
+
+static PyHamtNode *
+hamt_node_collision_new(int32_t hash, Py_ssize_t size)
+{
+    /* Create a new Collision node. */
+
+    PyHamtNode_Collision *node;
+    Py_ssize_t i;
+
+    assert(size >= 4);
+    assert(size % 2 == 0);
+
+    node = PyObject_GC_NewVar(
+        PyHamtNode_Collision, &_PyHamt_CollisionNode_Type, size);
+    if (node == NULL) {
+        return NULL;
+    }
+
+    for (i = 0; i < size; i++) {
+        node->c_array[i] = NULL;
+    }
+
+    Py_SIZE(node) = size;
+    node->c_hash = hash;
+
+    _PyObject_GC_TRACK(node);
+
+    return (PyHamtNode *)node;
+}
+
+static hamt_find_t
+hamt_node_collision_find_index(PyHamtNode_Collision *self, PyObject *key,
+                               Py_ssize_t *idx)
+{
+    /* Lookup `key` in the Collision node `self`.  Set the index of the
+       found key to 'idx'. */
+
+    Py_ssize_t i;
+    PyObject *el;
+
+    for (i = 0; i < Py_SIZE(self); i += 2) {
+        el = self->c_array[i];
+
+        assert(el != NULL);
+        int cmp = PyObject_RichCompareBool(key, el, Py_EQ);
+        if (cmp < 0) {
+            return F_ERROR;
+        }
+        if (cmp == 1) {
+            *idx = i;
+            return F_FOUND;
+        }
+    }
+
+    return F_NOT_FOUND;
+}
+
+static PyHamtNode *
+hamt_node_collision_assoc(PyHamtNode_Collision *self,
+                          uint32_t shift, int32_t hash,
+                          PyObject *key, PyObject *val, int* added_leaf)
+{
+    /* Set a new key to this level (currently a Collision node)
+       of the tree. */
+
+    if (hash == self->c_hash) {
+        /* The hash of the 'key' we are adding matches the hash of
+           other keys in this Collision node. */
+
+        Py_ssize_t key_idx = -1;
+        hamt_find_t found;
+        PyHamtNode_Collision *new_node;
+        Py_ssize_t i;
+
+        /* Let's try to lookup the new 'key', maybe we already have it. */
+        found = hamt_node_collision_find_index(self, key, &key_idx);
+        switch (found) {
+            case F_ERROR:
+                /* Exception. */
+                return NULL;
+
+            case F_NOT_FOUND:
+                /* This is a totally new key.  Clone the current node,
+                   add a new key/value to the cloned node. */
+
+                new_node = (PyHamtNode_Collision *)hamt_node_collision_new(
+                    self->c_hash, Py_SIZE(self) + 2);
+                if (new_node == NULL) {
+                    return NULL;
+                }
+
+                for (i = 0; i < Py_SIZE(self); i++) {
+                    Py_INCREF(self->c_array[i]);
+                    new_node->c_array[i] = self->c_array[i];
+                }
+
+                Py_INCREF(key);
+                new_node->c_array[i] = key;
+                Py_INCREF(val);
+                new_node->c_array[i + 1] = val;
+
+                *added_leaf = 1;
+                return (PyHamtNode *)new_node;
+
+            case F_FOUND:
+                /* There's a key which is equal to the key we are adding. */
+
+                assert(key_idx >= 0);
+                assert(key_idx < Py_SIZE(self));
+                Py_ssize_t val_idx = key_idx + 1;
+
+                if (self->c_array[val_idx] == val) {
+                    /* We're setting a key/value pair that's already set. */
+                    Py_INCREF(self);
+                    return (PyHamtNode *)self;
+                }
+
+                /* We need to replace old value for the key
+                   with a new value.  Create a new Collision node.*/
+                new_node = (PyHamtNode_Collision *)hamt_node_collision_new(
+                    self->c_hash, Py_SIZE(self));
+                if (new_node == NULL) {
+                    return NULL;
+                }
+
+                /* Copy all elements of the old node to the new one. */
+                for (i = 0; i < Py_SIZE(self); i++) {
+                    Py_INCREF(self->c_array[i]);
+                    new_node->c_array[i] = self->c_array[i];
+                }
+
+                /* Replace the old value with the new value for the our key. */
+                Py_DECREF(new_node->c_array[val_idx]);
+                Py_INCREF(val);
+                new_node->c_array[val_idx] = val;
+
+                return (PyHamtNode *)new_node;
+
+            default:
+                Py_UNREACHABLE();
+        }
+    }
+    else {
+        /* The hash of the new key is different from the hash that
+           all keys of this Collision node have.
+
+           Create a Bitmap node inplace with two children:
+           key/value pair that we're adding, and the Collision node
+           we're replacing on this tree level.
+        */
+
+        PyHamtNode_Bitmap *new_node;
+        PyHamtNode *assoc_res;
+
+        new_node = (PyHamtNode_Bitmap *)hamt_node_bitmap_new(2);
+        if (new_node == NULL) {
+            return NULL;
+        }
+        new_node->b_bitmap = hamt_bitpos(self->c_hash, shift);
+        Py_INCREF(self);
+        new_node->b_array[1] = (PyObject*) self;
+
+        assoc_res = hamt_node_bitmap_assoc(
+            new_node, shift, hash, key, val, added_leaf);
+        Py_DECREF(new_node);
+        return assoc_res;
+    }
+}
+
+static inline Py_ssize_t
+hamt_node_collision_count(PyHamtNode_Collision *node)
+{
+    return Py_SIZE(node) / 2;
+}
+
+static hamt_without_t
+hamt_node_collision_without(PyHamtNode_Collision *self,
+                            uint32_t shift, int32_t hash,
+                            PyObject *key,
+                            PyHamtNode **new_node)
+{
+    if (hash != self->c_hash) {
+        return W_NOT_FOUND;
+    }
+
+    Py_ssize_t key_idx = -1;
+    hamt_find_t found = hamt_node_collision_find_index(self, key, &key_idx);
+
+    switch (found) {
+        case F_ERROR:
+            return W_ERROR;
+
+        case F_NOT_FOUND:
+            return W_NOT_FOUND;
+
+        case F_FOUND:
+            assert(key_idx >= 0);
+            assert(key_idx < Py_SIZE(self));
+
+            Py_ssize_t new_count = hamt_node_collision_count(self) - 1;
+
+            if (new_count == 0) {
+                /* The node has only one key/value pair and it's for the
+                   key we're trying to delete.  So a new node will be empty
+                   after the removal.
+                */
+                return W_EMPTY;
+            }
+
+            if (new_count == 1) {
+                /* The node has two keys, and after deletion the
+                   new Collision node would have one.  Collision nodes
+                   with one key shouldn't exist, co convert it to a
+                   Bitmap node.
+                */
+                PyHamtNode_Bitmap *node = (PyHamtNode_Bitmap *)
+                    hamt_node_bitmap_new(2);
+                if (node == NULL) {
+                    return W_ERROR;
+                }
+
+                if (key_idx == 0) {
+                    Py_INCREF(self->c_array[2]);
+                    node->b_array[0] = self->c_array[2];
+                    Py_INCREF(self->c_array[3]);
+                    node->b_array[1] = self->c_array[3];
+                }
+                else {
+                    assert(key_idx == 2);
+                    Py_INCREF(self->c_array[0]);
+                    node->b_array[0] = self->c_array[0];
+                    Py_INCREF(self->c_array[1]);
+                    node->b_array[1] = self->c_array[1];
+                }
+
+                node->b_bitmap = hamt_bitpos(hash, shift);
+
+                *new_node = (PyHamtNode *)node;
+                return W_NEWNODE;
+            }
+
+            /* Allocate a new Collision node with capacity for one
+               less key/value pair */
+            PyHamtNode_Collision *new = (PyHamtNode_Collision *)
+                hamt_node_collision_new(
+                    self->c_hash, Py_SIZE(self) - 2);
+
+            /* Copy all other keys from `self` to `new` */
+            Py_ssize_t i;
+            for (i = 0; i < key_idx; i++) {
+                Py_INCREF(self->c_array[i]);
+                new->c_array[i] = self->c_array[i];
+            }
+            for (i = key_idx + 2; i < Py_SIZE(self); i++) {
+                Py_INCREF(self->c_array[i]);
+                new->c_array[i - 2] = self->c_array[i];
+            }
+
+            *new_node = (PyHamtNode*)new;
+            return W_NEWNODE;
+
+        default:
+            Py_UNREACHABLE();
+    }
+}
+
+static hamt_find_t
+hamt_node_collision_find(PyHamtNode_Collision *self,
+                         uint32_t shift, int32_t hash,
+                         PyObject *key, PyObject **val)
+{
+    /* Lookup `key` in the Collision node `self`.  Set the value
+       for the found key to 'val'. */
+
+    Py_ssize_t idx = -1;
+    hamt_find_t res;
+
+    res = hamt_node_collision_find_index(self, key, &idx);
+    if (res == F_ERROR || res == F_NOT_FOUND) {
+        return res;
+    }
+
+    assert(idx >= 0);
+    assert(idx + 1 < Py_SIZE(self));
+
+    *val = self->c_array[idx + 1];
+    assert(*val != NULL);
+
+    return F_FOUND;
+}
+
+
+static int
+hamt_node_collision_traverse(PyHamtNode_Collision *self,
+                             visitproc visit, void *arg)
+{
+    /* Collision's tp_traverse */
+
+    Py_ssize_t i;
+
+    for (i = Py_SIZE(self); --i >= 0; ) {
+        Py_VISIT(self->c_array[i]);
+    }
+
+    return 0;
+}
+
+static void
+hamt_node_collision_dealloc(PyHamtNode_Collision *self)
+{
+    /* Collision's tp_dealloc */
+
+    Py_ssize_t len = Py_SIZE(self);
+
+    PyObject_GC_UnTrack(self);
+    Py_TRASHCAN_SAFE_BEGIN(self)
+
+    if (len > 0) {
+
+        while (--len >= 0) {
+            Py_XDECREF(self->c_array[len]);
+        }
+    }
+
+    Py_TYPE(self)->tp_free((PyObject *)self);
+    Py_TRASHCAN_SAFE_END(self)
+}
+
+#ifdef Py_DEBUG
+static int
+hamt_node_collision_dump(PyHamtNode_Collision *node,
+                         _PyUnicodeWriter *writer, int level)
+{
+    /* Debug build: __dump__() method implementation for Collision nodes. */
+
+    Py_ssize_t i;
+
+    if (_hamt_dump_ident(writer, level + 1)) {
+        goto error;
+    }
+
+    if (_hamt_dump_format(writer, "CollisionNode(size=%zd id=%p):\n",
+                          Py_SIZE(node), node))
+    {
+        goto error;
+    }
+
+    for (i = 0; i < Py_SIZE(node); i += 2) {
+        PyObject *key = node->c_array[i];
+        PyObject *val = node->c_array[i + 1];
+
+        if (_hamt_dump_ident(writer, level + 2)) {
+            goto error;
+        }
+
+        if (_hamt_dump_format(writer, "%R: %R\n", key, val)) {
+            goto error;
+        }
+    }
+
+    return 0;
+error:
+    return -1;
+}
+#endif  /* Py_DEBUG */
+
+
+/////////////////////////////////// Array Node
+
+
+static PyHamtNode *
+hamt_node_array_new(Py_ssize_t count)
+{
+    Py_ssize_t i;
+
+    PyHamtNode_Array *node = PyObject_GC_New(
+        PyHamtNode_Array, &_PyHamt_ArrayNode_Type);
+    if (node == NULL) {
+        return NULL;
+    }
+
+    for (i = 0; i < HAMT_ARRAY_NODE_SIZE; i++) {
+        node->a_array[i] = NULL;
+    }
+
+    node->a_count = count;
+
+    _PyObject_GC_TRACK(node);
+    return (PyHamtNode *)node;
+}
+
+static PyHamtNode_Array *
+hamt_node_array_clone(PyHamtNode_Array *node)
+{
+    PyHamtNode_Array *clone;
+    Py_ssize_t i;
+
+    VALIDATE_ARRAY_NODE(node)
+
+    /* Create a new Array node. */
+    clone = (PyHamtNode_Array *)hamt_node_array_new(node->a_count);
+    if (clone == NULL) {
+        return NULL;
+    }
+
+    /* Copy all elements from the current Array node to the new one. */
+    for (i = 0; i < HAMT_ARRAY_NODE_SIZE; i++) {
+        Py_XINCREF(node->a_array[i]);
+        clone->a_array[i] = node->a_array[i];
+    }
+
+    VALIDATE_ARRAY_NODE(clone)
+    return clone;
+}
+
+static PyHamtNode *
+hamt_node_array_assoc(PyHamtNode_Array *self,
+                      uint32_t shift, int32_t hash,
+                      PyObject *key, PyObject *val, int* added_leaf)
+{
+    /* Set a new key to this level (currently a Collision node)
+       of the tree.
+
+       Array nodes don't store values, they can only point to
+       other nodes.  They are simple arrays of 32 BaseNode pointers/
+     */
+
+    uint32_t idx = hamt_mask(hash, shift);
+    PyHamtNode *node = self->a_array[idx];
+    PyHamtNode *child_node;
+    PyHamtNode_Array *new_node;
+    Py_ssize_t i;
+
+    if (node == NULL) {
+        /* There's no child node for the given hash.  Create a new
+           Bitmap node for this key. */
+
+        PyHamtNode_Bitmap *empty = NULL;
+
+        /* Get an empty Bitmap node to work with. */
+        empty = (PyHamtNode_Bitmap *)hamt_node_bitmap_new(0);
+        if (empty == NULL) {
+            return NULL;
+        }
+
+        /* Set key/val to the newly created empty Bitmap, thus
+           creating a new Bitmap node with our key/value pair. */
+        child_node = hamt_node_bitmap_assoc(
+            empty,
+            shift + 5, hash, key, val, added_leaf);
+        Py_DECREF(empty);
+        if (child_node == NULL) {
+            return NULL;
+        }
+
+        /* Create a new Array node. */
+        new_node = (PyHamtNode_Array *)hamt_node_array_new(self->a_count + 1);
+        if (new_node == NULL) {
+            Py_DECREF(child_node);
+            return NULL;
+        }
+
+        /* Copy all elements from the current Array node to the
+           new one. */
+        for (i = 0; i < HAMT_ARRAY_NODE_SIZE; i++) {
+            Py_XINCREF(self->a_array[i]);
+            new_node->a_array[i] = self->a_array[i];
+        }
+
+        assert(new_node->a_array[idx] == NULL);
+        new_node->a_array[idx] = child_node;  /* borrow */
+        VALIDATE_ARRAY_NODE(new_node)
+    }
+    else {
+        /* There's a child node for the given hash.
+           Set the key to it./ */
+        child_node = hamt_node_assoc(
+            node, shift + 5, hash, key, val, added_leaf);
+        if (child_node == (PyHamtNode *)self) {
+            Py_DECREF(child_node);
+            return (PyHamtNode *)self;
+        }
+
+        new_node = hamt_node_array_clone(self);
+        if (new_node == NULL) {
+            Py_DECREF(child_node);
+            return NULL;
+        }
+
+        Py_SETREF(new_node->a_array[idx], child_node);  /* borrow */
+        VALIDATE_ARRAY_NODE(new_node)
+    }
+
+    return (PyHamtNode *)new_node;
+}
+
+static hamt_without_t
+hamt_node_array_without(PyHamtNode_Array *self,
+                        uint32_t shift, int32_t hash,
+                        PyObject *key,
+                        PyHamtNode **new_node)
+{
+    uint32_t idx = hamt_mask(hash, shift);
+    PyHamtNode *node = self->a_array[idx];
+
+    if (node == NULL) {
+        return W_NOT_FOUND;
+    }
+
+    PyHamtNode *sub_node = NULL;
+    hamt_without_t res = hamt_node_without(
+        (PyHamtNode *)node,
+        shift + 5, hash, key, &sub_node);
+
+    switch (res) {
+        case W_NOT_FOUND:
+        case W_ERROR:
+            assert(sub_node == NULL);
+            return res;
+
+        case W_NEWNODE: {
+            /* We need to replace a node at the `idx` index.
+               Clone this node and replace.
+            */
+            assert(sub_node != NULL);
+
+            PyHamtNode_Array *clone = hamt_node_array_clone(self);
+            if (clone == NULL) {
+                Py_DECREF(sub_node);
+                return W_ERROR;
+            }
+
+            Py_SETREF(clone->a_array[idx], sub_node);  /* borrow */
+            *new_node = (PyHamtNode*)clone;  /* borrow */
+            return W_NEWNODE;
+        }
+
+        case W_EMPTY: {
+            assert(sub_node == NULL);
+            /* We need to remove a node at the `idx` index.
+               Calculate the size of the replacement Array node.
+            */
+            Py_ssize_t new_count = self->a_count - 1;
+
+            if (new_count == 0) {
+                return W_EMPTY;
+            }
+
+            if (new_count >= 16) {
+                /* We convert Bitmap nodes to Array nodes, when a
+                   Bitmap node needs to store more than 15 key/value
+                   pairs.  So we will create a new Array node if we
+                   the number of key/values after deletion is still
+                   greater than 15.
+                */
+
+                PyHamtNode_Array *new = hamt_node_array_clone(self);
+                if (new == NULL) {
+                    return W_ERROR;
+                }
+                new->a_count = new_count;
+                Py_CLEAR(new->a_array[idx]);
+
+                *new_node = (PyHamtNode*)new;  /* borrow */
+                return W_NEWNODE;
+            }
+
+            /* New Array node would have less than 16 key/value
+               pairs.  We need to create a replacement Bitmap node. */
+
+            Py_ssize_t bitmap_size = new_count * 2;
+            uint32_t bitmap = 0;
+
+            PyHamtNode_Bitmap *new = (PyHamtNode_Bitmap *)
+                hamt_node_bitmap_new(bitmap_size);
+            if (new == NULL) {
+                return W_ERROR;
+            }
+
+            Py_ssize_t new_i = 0;
+            for (uint32_t i = 0; i < HAMT_ARRAY_NODE_SIZE; i++) {
+                if (i == idx) {
+                    /* Skip the node we are deleting. */
+                    continue;
+                }
+
+                PyHamtNode *node = self->a_array[i];
+                if (node == NULL) {
+                    /* Skip any missing nodes. */
+                    continue;
+                }
+
+                bitmap |= 1 << i;
+
+                if (IS_BITMAP_NODE(node)) {
+                    PyHamtNode_Bitmap *child = (PyHamtNode_Bitmap *)node;
+
+                    if (hamt_node_bitmap_count(child) == 1 &&
+                            child->b_array[0] != NULL)
+                    {
+                        /* node is a Bitmap with one key/value pair, just
+                           merge it into the new Bitmap node we're building.
+
+                           Note that we don't inline Bitmap nodes that
+                           have a NULL key -- those nodes point to another
+                           tree level, and we cannot simply move tree levels
+                           up or down.
+                        */
+                        PyObject *key = child->b_array[0];
+                        PyObject *val = child->b_array[1];
+
+                        Py_INCREF(key);
+                        new->b_array[new_i] = key;
+                        Py_INCREF(val);
+                        new->b_array[new_i + 1] = val;
+                    }
+                    else {
+                        new->b_array[new_i] = NULL;
+                        Py_INCREF(node);
+                        new->b_array[new_i + 1] = (PyObject*)node;
+                    }
+                }
+                else {
+
+#ifdef Py_DEBUG
+                    if (IS_COLLISION_NODE(node)) {
+                        Py_ssize_t child_count = hamt_node_collision_count(
+                            (PyHamtNode_Collision*)node);
+                        assert(child_count > 1);
+                    }
+                    else if (IS_ARRAY_NODE(node)) {
+                        assert(((PyHamtNode_Array*)node)->a_count >= 16);
+                    }
+#endif
+
+                    /* Just copy the node into our new Bitmap */
+                    new->b_array[new_i] = NULL;
+                    Py_INCREF(node);
+                    new->b_array[new_i + 1] = (PyObject*)node;
+                }
+
+                new_i += 2;
+            }
+
+            new->b_bitmap = bitmap;
+            *new_node = (PyHamtNode*)new;  /* borrow */
+            return W_NEWNODE;
+        }
+
+        default:
+            Py_UNREACHABLE();
+    }
+}
+
+static hamt_find_t
+hamt_node_array_find(PyHamtNode_Array *self,
+                     uint32_t shift, int32_t hash,
+                     PyObject *key, PyObject **val)
+{
+    /* Lookup `key` in the Array node `self`.  Set the value
+       for the found key to 'val'. */
+
+    uint32_t idx = hamt_mask(hash, shift);
+    PyHamtNode *node;
+
+    node = self->a_array[idx];
+    if (node == NULL) {
+        return F_NOT_FOUND;
+    }
+
+    /* Dispatch to the generic hamt_node_find */
+    return hamt_node_find(node, shift + 5, hash, key, val);
+}
+
+static int
+hamt_node_array_traverse(PyHamtNode_Array *self,
+                         visitproc visit, void *arg)
+{
+    /* Array's tp_traverse */
+
+    Py_ssize_t i;
+
+    for (i = 0; i < HAMT_ARRAY_NODE_SIZE; i++) {
+        Py_VISIT(self->a_array[i]);
+    }
+
+    return 0;
+}
+
+static void
+hamt_node_array_dealloc(PyHamtNode_Array *self)
+{
+    /* Array's tp_dealloc */
+
+    Py_ssize_t i;
+
+    PyObject_GC_UnTrack(self);
+    Py_TRASHCAN_SAFE_BEGIN(self)
+
+    for (i = 0; i < HAMT_ARRAY_NODE_SIZE; i++) {
+        Py_XDECREF(self->a_array[i]);
+    }
+
+    Py_TYPE(self)->tp_free((PyObject *)self);
+    Py_TRASHCAN_SAFE_END(self)
+}
+
+#ifdef Py_DEBUG
+static int
+hamt_node_array_dump(PyHamtNode_Array *node,
+                     _PyUnicodeWriter *writer, int level)
+{
+    /* Debug build: __dump__() method implementation for Array nodes. */
+
+    Py_ssize_t i;
+
+    if (_hamt_dump_ident(writer, level + 1)) {
+        goto error;
+    }
+
+    if (_hamt_dump_format(writer, "ArrayNode(id=%p):\n", node)) {
+        goto error;
+    }
+
+    for (i = 0; i < HAMT_ARRAY_NODE_SIZE; i++) {
+        if (node->a_array[i] == NULL) {
+            continue;
+        }
+
+        if (_hamt_dump_ident(writer, level + 2)) {
+            goto error;
+        }
+
+        if (_hamt_dump_format(writer, "%d::\n", i)) {
+            goto error;
+        }
+
+        if (hamt_node_dump(node->a_array[i], writer, level + 1)) {
+            goto error;
+        }
+
+        if (_hamt_dump_format(writer, "\n")) {
+            goto error;
+        }
+    }
+
+    return 0;
+error:
+    return -1;
+}
+#endif  /* Py_DEBUG */
+
+
+/////////////////////////////////// Node Dispatch
+
+
+static PyHamtNode *
+hamt_node_assoc(PyHamtNode *node,
+                uint32_t shift, int32_t hash,
+                PyObject *key, PyObject *val, int* added_leaf)
+{
+    /* Set key/value to the 'node' starting with the given shift/hash.
+       Return a new node, or the same node if key/value already
+       set.
+
+       added_leaf will be set to 1 if key/value wasn't in the
+       tree before.
+
+       This method automatically dispatches to the suitable
+       hamt_node_{nodetype}_assoc method.
+    */
+
+    if (IS_BITMAP_NODE(node)) {
+        return hamt_node_bitmap_assoc(
+            (PyHamtNode_Bitmap *)node,
+            shift, hash, key, val, added_leaf);
+    }
+    else if (IS_ARRAY_NODE(node)) {
+        return hamt_node_array_assoc(
+            (PyHamtNode_Array *)node,
+            shift, hash, key, val, added_leaf);
+    }
+    else {
+        assert(IS_COLLISION_NODE(node));
+        return hamt_node_collision_assoc(
+            (PyHamtNode_Collision *)node,
+            shift, hash, key, val, added_leaf);
+    }
+}
+
+static hamt_without_t
+hamt_node_without(PyHamtNode *node,
+                  uint32_t shift, int32_t hash,
+                  PyObject *key,
+                  PyHamtNode **new_node)
+{
+    if (IS_BITMAP_NODE(node)) {
+        return hamt_node_bitmap_without(
+            (PyHamtNode_Bitmap *)node,
+            shift, hash, key,
+            new_node);
+    }
+    else if (IS_ARRAY_NODE(node)) {
+        return hamt_node_array_without(
+            (PyHamtNode_Array *)node,
+            shift, hash, key,
+            new_node);
+    }
+    else {
+        assert(IS_COLLISION_NODE(node));
+        return hamt_node_collision_without(
+            (PyHamtNode_Collision *)node,
+            shift, hash, key,
+            new_node);
+    }
+}
+
+static hamt_find_t
+hamt_node_find(PyHamtNode *node,
+               uint32_t shift, int32_t hash,
+               PyObject *key, PyObject **val)
+{
+    /* Find the key in the node starting with the given shift/hash.
+
+       If a value is found, the result will be set to F_FOUND, and
+       *val will point to the found value object.
+
+       If a value wasn't found, the result will be set to F_NOT_FOUND.
+
+       If an exception occurs during the call, the result will be F_ERROR.
+
+       This method automatically dispatches to the suitable
+       hamt_node_{nodetype}_find method.
+    */
+
+    if (IS_BITMAP_NODE(node)) {
+        return hamt_node_bitmap_find(
+            (PyHamtNode_Bitmap *)node,
+            shift, hash, key, val);
+
+    }
+    else if (IS_ARRAY_NODE(node)) {
+        return hamt_node_array_find(
+            (PyHamtNode_Array *)node,
+            shift, hash, key, val);
+    }
+    else {
+        assert(IS_COLLISION_NODE(node));
+        return hamt_node_collision_find(
+            (PyHamtNode_Collision *)node,
+            shift, hash, key, val);
+    }
+}
+
+#ifdef Py_DEBUG
+static int
+hamt_node_dump(PyHamtNode *node,
+               _PyUnicodeWriter *writer, int level)
+{
+    /* Debug build: __dump__() method implementation for a node.
+
+       This method automatically dispatches to the suitable
+       hamt_node_{nodetype})_dump method.
+    */
+
+    if (IS_BITMAP_NODE(node)) {
+        return hamt_node_bitmap_dump(
+            (PyHamtNode_Bitmap *)node, writer, level);
+    }
+    else if (IS_ARRAY_NODE(node)) {
+        return hamt_node_array_dump(
+            (PyHamtNode_Array *)node, writer, level);
+    }
+    else {
+        assert(IS_COLLISION_NODE(node));
+        return hamt_node_collision_dump(
+            (PyHamtNode_Collision *)node, writer, level);
+    }
+}
+#endif  /* Py_DEBUG */
+
+
+/////////////////////////////////// Iterators: Machinery
+
+
+static hamt_iter_t
+hamt_iterator_next(PyHamtIteratorState *iter, PyObject **key, PyObject **val);
+
+
+static void
+hamt_iterator_init(PyHamtIteratorState *iter, PyHamtNode *root)
+{
+    for (uint32_t i = 0; i < _Py_HAMT_MAX_TREE_DEPTH; i++) {
+        iter->i_nodes[i] = NULL;
+        iter->i_pos[i] = 0;
+    }
+
+    iter->i_level = 0;
+
+    /* Note: we don't incref/decref nodes in i_nodes. */
+    iter->i_nodes[0] = root;
+}
+
+static hamt_iter_t
+hamt_iterator_bitmap_next(PyHamtIteratorState *iter,
+                          PyObject **key, PyObject **val)
+{
+    int8_t level = iter->i_level;
+
+    PyHamtNode_Bitmap *node = (PyHamtNode_Bitmap *)(iter->i_nodes[level]);
+    Py_ssize_t pos = iter->i_pos[level];
+
+    if (pos + 1 >= Py_SIZE(node)) {
+#ifdef Py_DEBUG
+        assert(iter->i_level >= 0);
+        iter->i_nodes[iter->i_level] = NULL;
+#endif
+        iter->i_level--;
+        return hamt_iterator_next(iter, key, val);
+    }
+
+    if (node->b_array[pos] == NULL) {
+        iter->i_pos[level] = pos + 2;
+
+        int8_t next_level = level + 1;
+        assert(next_level < _Py_HAMT_MAX_TREE_DEPTH);
+        iter->i_level = next_level;
+        iter->i_pos[next_level] = 0;
+        iter->i_nodes[next_level] = (PyHamtNode *)
+            node->b_array[pos + 1];
+
+        return hamt_iterator_next(iter, key, val);
+    }
+
+    *key = node->b_array[pos];
+    *val = node->b_array[pos + 1];
+    iter->i_pos[level] = pos + 2;
+    return I_ITEM;
+}
+
+static hamt_iter_t
+hamt_iterator_collision_next(PyHamtIteratorState *iter,
+                             PyObject **key, PyObject **val)
+{
+    int8_t level = iter->i_level;
+
+    PyHamtNode_Collision *node = (PyHamtNode_Collision *)(iter->i_nodes[level]);
+    Py_ssize_t pos = iter->i_pos[level];
+
+    if (pos + 1 >= Py_SIZE(node)) {
+#ifdef Py_DEBUG
+        assert(iter->i_level >= 0);
+        iter->i_nodes[iter->i_level] = NULL;
+#endif
+        iter->i_level--;
+        return hamt_iterator_next(iter, key, val);
+    }
+
+    *key = node->c_array[pos];
+    *val = node->c_array[pos + 1];
+    iter->i_pos[level] = pos + 2;
+    return I_ITEM;
+}
+
+static hamt_iter_t
+hamt_iterator_array_next(PyHamtIteratorState *iter,
+                         PyObject **key, PyObject **val)
+{
+    int8_t level = iter->i_level;
+
+    PyHamtNode_Array *node = (PyHamtNode_Array *)(iter->i_nodes[level]);
+    Py_ssize_t pos = iter->i_pos[level];
+
+    if (pos >= HAMT_ARRAY_NODE_SIZE) {
+#ifdef Py_DEBUG
+        assert(iter->i_level >= 0);
+        iter->i_nodes[iter->i_level] = NULL;
+#endif
+        iter->i_level--;
+        return hamt_iterator_next(iter, key, val);
+    }
+
+    for (Py_ssize_t i = pos; i < HAMT_ARRAY_NODE_SIZE; i++) {
+        if (node->a_array[i] != NULL) {
+            iter->i_pos[level] = i + 1;
+
+            int8_t next_level = level + 1;
+            assert(next_level < _Py_HAMT_MAX_TREE_DEPTH);
+            iter->i_pos[next_level] = 0;
+            iter->i_nodes[next_level] = node->a_array[i];
+            iter->i_level = next_level;
+
+            return hamt_iterator_next(iter, key, val);
+        }
+    }
+
+#ifdef Py_DEBUG
+        assert(iter->i_level >= 0);
+        iter->i_nodes[iter->i_level] = NULL;
+#endif
+
+    iter->i_level--;
+    return hamt_iterator_next(iter, key, val);
+}
+
+static hamt_iter_t
+hamt_iterator_next(PyHamtIteratorState *iter, PyObject **key, PyObject **val)
+{
+    if (iter->i_level < 0) {
+        return I_END;
+    }
+
+    assert(iter->i_level < _Py_HAMT_MAX_TREE_DEPTH);
+
+    PyHamtNode *current = iter->i_nodes[iter->i_level];
+
+    if (IS_BITMAP_NODE(current)) {
+        return hamt_iterator_bitmap_next(iter, key, val);
+    }
+    else if (IS_ARRAY_NODE(current)) {
+        return hamt_iterator_array_next(iter, key, val);
+    }
+    else {
+        assert(IS_COLLISION_NODE(current));
+        return hamt_iterator_collision_next(iter, key, val);
+    }
+}
+
+
+/////////////////////////////////// HAMT high-level functions
+
+
+PyHamtObject *
+_PyHamt_Assoc(PyHamtObject *o, PyObject *key, PyObject *val)
+{
+    int32_t key_hash;
+    int added_leaf = 0;
+    PyHamtNode *new_root;
+    PyHamtObject *new_o;
+
+    key_hash = hamt_hash(key);
+    if (key_hash == -1) {
+        return NULL;
+    }
+
+    new_root = hamt_node_assoc(
+        (PyHamtNode *)(o->h_root),
+        0, key_hash, key, val, &added_leaf);
+    if (new_root == NULL) {
+        return NULL;
+    }
+
+    if (new_root == o->h_root) {
+        Py_DECREF(new_root);
+        Py_INCREF(o);
+        return o;
+    }
+
+    new_o = hamt_alloc();
+    if (new_o == NULL) {
+        Py_DECREF(new_root);
+        return NULL;
+    }
+
+    new_o->h_root = new_root;  /* borrow */
+    new_o->h_count = added_leaf ? o->h_count + 1 : o->h_count;
+
+    return new_o;
+}
+
+PyHamtObject *
+_PyHamt_Without(PyHamtObject *o, PyObject *key)
+{
+    int32_t key_hash = hamt_hash(key);
+    if (key_hash == -1) {
+        return NULL;
+    }
+
+    PyHamtNode *new_root;
+
+    hamt_without_t res = hamt_node_without(
+        (PyHamtNode *)(o->h_root),
+        0, key_hash, key,
+        &new_root);
+
+    switch (res) {
+        case W_ERROR:
+            return NULL;
+        case W_EMPTY:
+            return _PyHamt_New();
+        case W_NOT_FOUND:
+            Py_INCREF(o);
+            return o;
+        case W_NEWNODE: {
+            PyHamtObject *new_o = hamt_alloc();
+            if (new_o == NULL) {
+                Py_DECREF(new_root);
+                return NULL;
+            }
+
+            new_o->h_root = new_root;  /* borrow */
+            new_o->h_count = o->h_count - 1;
+            assert(new_o->h_count >= 0);
+            return new_o;
+        }
+        default:
+            Py_UNREACHABLE();
+    }
+}
+
+static hamt_find_t
+hamt_find(PyHamtObject *o, PyObject *key, PyObject **val)
+{
+    if (o->h_count == 0) {
+        return F_NOT_FOUND;
+    }
+
+    int32_t key_hash = hamt_hash(key);
+    if (key_hash == -1) {
+        return F_ERROR;
+    }
+
+    return hamt_node_find(o->h_root, 0, key_hash, key, val);
+}
+
+
+int
+_PyHamt_Find(PyHamtObject *o, PyObject *key, PyObject **val)
+{
+    hamt_find_t res = hamt_find(o, key, val);
+    switch (res) {
+        case F_ERROR:
+            return -1;
+        case F_NOT_FOUND:
+            return 0;
+        case F_FOUND:
+            return 1;
+        default:
+            Py_UNREACHABLE();
+    }
+}
+
+
+int
+_PyHamt_Eq(PyHamtObject *v, PyHamtObject *w)
+{
+    if (v == w) {
+        return 1;
+    }
+
+    if (v->h_count != w->h_count) {
+        return 0;
+    }
+
+    PyHamtIteratorState iter;
+    hamt_iter_t iter_res;
+    hamt_find_t find_res;
+    PyObject *v_key;
+    PyObject *v_val;
+    PyObject *w_val;
+
+    hamt_iterator_init(&iter, v->h_root);
+
+    do {
+        iter_res = hamt_iterator_next(&iter, &v_key, &v_val);
+        if (iter_res == I_ITEM) {
+            find_res = hamt_find(w, v_key, &w_val);
+            switch (find_res) {
+                case F_ERROR:
+                    return -1;
+
+                case F_NOT_FOUND:
+                    return 0;
+
+                case F_FOUND: {
+                    int cmp = PyObject_RichCompareBool(v_val, w_val, Py_EQ);
+                    if (cmp < 0) {
+                        return -1;
+                    }
+                    if (cmp == 0) {
+                        return 0;
+                    }
+                }
+            }
+        }
+    } while (iter_res != I_END);
+
+    return 1;
+}
+
+Py_ssize_t
+_PyHamt_Len(PyHamtObject *o)
+{
+    return o->h_count;
+}
+
+static PyHamtObject *
+hamt_alloc(void)
+{
+    PyHamtObject *o;
+    o = PyObject_GC_New(PyHamtObject, &_PyHamt_Type);
+    if (o == NULL) {
+        return NULL;
+    }
+    o->h_weakreflist = NULL;
+    PyObject_GC_Track(o);
+    return o;
+}
+
+PyHamtObject *
+_PyHamt_New(void)
+{
+    if (_empty_hamt != NULL) {
+        /* HAMT is an immutable object so we can easily cache an
+           empty instance. */
+        Py_INCREF(_empty_hamt);
+        return _empty_hamt;
+    }
+
+    PyHamtObject *o = hamt_alloc();
+    if (o == NULL) {
+        return NULL;
+    }
+
+    o->h_root = hamt_node_bitmap_new(0);
+    if (o->h_root == NULL) {
+        Py_DECREF(o);
+        return NULL;
+    }
+
+    o->h_count = 0;
+
+    if (_empty_hamt == NULL) {
+        Py_INCREF(o);
+        _empty_hamt = o;
+    }
+
+    return o;
+}
+
+#ifdef Py_DEBUG
+static PyObject *
+hamt_dump(PyHamtObject *self)
+{
+    _PyUnicodeWriter writer;
+
+    _PyUnicodeWriter_Init(&writer);
+
+    if (_hamt_dump_format(&writer, "HAMT(len=%zd):\n", self->h_count)) {
+        goto error;
+    }
+
+    if (hamt_node_dump(self->h_root, &writer, 0)) {
+        goto error;
+    }
+
+    return _PyUnicodeWriter_Finish(&writer);
+
+error:
+    _PyUnicodeWriter_Dealloc(&writer);
+    return NULL;
+}
+#endif  /* Py_DEBUG */
+
+
+/////////////////////////////////// Iterators: Shared Iterator Implementation
+
+
+static int
+hamt_baseiter_tp_clear(PyHamtIterator *it)
+{
+    Py_CLEAR(it->hi_obj);
+    return 0;
+}
+
+static void
+hamt_baseiter_tp_dealloc(PyHamtIterator *it)
+{
+    PyObject_GC_UnTrack(it);
+    (void)hamt_baseiter_tp_clear(it);
+    PyObject_GC_Del(it);
+}
+
+static int
+hamt_baseiter_tp_traverse(PyHamtIterator *it, visitproc visit, void *arg)
+{
+    Py_VISIT(it->hi_obj);
+    return 0;
+}
+
+static PyObject *
+hamt_baseiter_tp_iternext(PyHamtIterator *it)
+{
+    PyObject *key;
+    PyObject *val;
+    hamt_iter_t res = hamt_iterator_next(&it->hi_iter, &key, &val);
+
+    switch (res) {
+        case I_END:
+            PyErr_SetNone(PyExc_StopIteration);
+            return NULL;
+
+        case I_ITEM: {
+            return (*(it->hi_yield))(key, val);
+        }
+
+        default: {
+            Py_UNREACHABLE();
+        }
+    }
+}
+
+static Py_ssize_t
+hamt_baseiter_tp_len(PyHamtIterator *it)
+{
+    return it->hi_obj->h_count;
+}
+
+static PyMappingMethods PyHamtIterator_as_mapping = {
+    (lenfunc)hamt_baseiter_tp_len,
+};
+
+static PyObject *
+hamt_baseiter_new(PyTypeObject *type, binaryfunc yield, PyHamtObject *o)
+{
+    PyHamtIterator *it = PyObject_GC_New(PyHamtIterator, type);
+    if (it == NULL) {
+        return NULL;
+    }
+
+    Py_INCREF(o);
+    it->hi_obj = o;
+    it->hi_yield = yield;
+
+    hamt_iterator_init(&it->hi_iter, o->h_root);
+
+    return (PyObject*)it;
+}
+
+#define ITERATOR_TYPE_SHARED_SLOTS                              \
+    .tp_basicsize = sizeof(PyHamtIterator),                     \
+    .tp_itemsize = 0,                                           \
+    .tp_as_mapping = &PyHamtIterator_as_mapping,                \
+    .tp_dealloc = (destructor)hamt_baseiter_tp_dealloc,         \
+    .tp_getattro = PyObject_GenericGetAttr,                     \
+    .tp_flags = Py_TPFLAGS_DEFAULT | Py_TPFLAGS_HAVE_GC,        \
+    .tp_traverse = (traverseproc)hamt_baseiter_tp_traverse,     \
+    .tp_clear = (inquiry)hamt_baseiter_tp_clear,                \
+    .tp_iter = PyObject_SelfIter,                               \
+    .tp_iternext = (iternextfunc)hamt_baseiter_tp_iternext,
+
+
+/////////////////////////////////// _PyHamtItems_Type
+
+
+PyTypeObject _PyHamtItems_Type = {
+    PyVarObject_HEAD_INIT(NULL, 0)
+    "items",
+    ITERATOR_TYPE_SHARED_SLOTS
+};
+
+static PyObject *
+hamt_iter_yield_items(PyObject *key, PyObject *val)
+{
+    return PyTuple_Pack(2, key, val);
+}
+
+PyObject *
+_PyHamt_NewIterItems(PyHamtObject *o)
+{
+    return hamt_baseiter_new(
+        &_PyHamtItems_Type, hamt_iter_yield_items, o);
+}
+
+
+/////////////////////////////////// _PyHamtKeys_Type
+
+
+PyTypeObject _PyHamtKeys_Type = {
+    PyVarObject_HEAD_INIT(NULL, 0)
+    "keys",
+    ITERATOR_TYPE_SHARED_SLOTS
+};
+
+static PyObject *
+hamt_iter_yield_keys(PyObject *key, PyObject *val)
+{
+    Py_INCREF(key);
+    return key;
+}
+
+PyObject *
+_PyHamt_NewIterKeys(PyHamtObject *o)
+{
+    return hamt_baseiter_new(
+        &_PyHamtKeys_Type, hamt_iter_yield_keys, o);
+}
+
+
+/////////////////////////////////// _PyHamtValues_Type
+
+
+PyTypeObject _PyHamtValues_Type = {
+    PyVarObject_HEAD_INIT(NULL, 0)
+    "values",
+    ITERATOR_TYPE_SHARED_SLOTS
+};
+
+static PyObject *
+hamt_iter_yield_values(PyObject *key, PyObject *val)
+{
+    Py_INCREF(val);
+    return val;
+}
+
+PyObject *
+_PyHamt_NewIterValues(PyHamtObject *o)
+{
+    return hamt_baseiter_new(
+        &_PyHamtValues_Type, hamt_iter_yield_values, o);
+}
+
+
+/////////////////////////////////// _PyHamt_Type
+
+
+#ifdef Py_DEBUG
+static PyObject *
+hamt_dump(PyHamtObject *self);
+#endif
+
+
+static PyObject *
+hamt_tp_new(PyTypeObject *type, PyObject *args, PyObject *kwds)
+{
+    return (PyObject*)_PyHamt_New();
+}
+
+static int
+hamt_tp_clear(PyHamtObject *self)
+{
+    Py_CLEAR(self->h_root);
+    return 0;
+}
+
+
+static int
+hamt_tp_traverse(PyHamtObject *self, visitproc visit, void *arg)
+{
+    Py_VISIT(self->h_root);
+    return 0;
+}
+
+static void
+hamt_tp_dealloc(PyHamtObject *self)
+{
+    PyObject_GC_UnTrack(self);
+    if (self->h_weakreflist != NULL) {
+        PyObject_ClearWeakRefs((PyObject*)self);
+    }
+    (void)hamt_tp_clear(self);
+    Py_TYPE(self)->tp_free(self);
+}
+
+
+static PyObject *
+hamt_tp_richcompare(PyObject *v, PyObject *w, int op)
+{
+    if (!PyHamt_Check(v) || !PyHamt_Check(w) || (op != Py_EQ && op != Py_NE)) {
+        Py_RETURN_NOTIMPLEMENTED;
+    }
+
+    int res = _PyHamt_Eq((PyHamtObject *)v, (PyHamtObject *)w);
+    if (res < 0) {
+        return NULL;
+    }
+
+    if (op == Py_NE) {
+        res = !res;
+    }
+
+    if (res) {
+        Py_RETURN_TRUE;
+    }
+    else {
+        Py_RETURN_FALSE;
+    }
+}
+
+static int
+hamt_tp_contains(PyHamtObject *self, PyObject *key)
+{
+    PyObject *val;
+    return _PyHamt_Find(self, key, &val);
+}
+
+static PyObject *
+hamt_tp_subscript(PyHamtObject *self, PyObject *key)
+{
+    PyObject *val;
+    hamt_find_t res = hamt_find(self, key, &val);
+    switch (res) {
+        case F_ERROR:
+            return NULL;
+        case F_FOUND:
+            Py_INCREF(val);
+            return val;
+        case F_NOT_FOUND:
+            PyErr_SetObject(PyExc_KeyError, key);
+            return NULL;
+        default:
+            Py_UNREACHABLE();
+    }
+}
+
+static Py_ssize_t
+hamt_tp_len(PyHamtObject *self)
+{
+    return _PyHamt_Len(self);
+}
+
+static PyObject *
+hamt_tp_iter(PyHamtObject *self)
+{
+    return _PyHamt_NewIterKeys(self);
+}
+
+static PyObject *
+hamt_py_set(PyHamtObject *self, PyObject *args)
+{
+    PyObject *key;
+    PyObject *val;
+
+    if (!PyArg_UnpackTuple(args, "set", 2, 2, &key, &val)) {
+        return NULL;
+    }
+
+    return (PyObject *)_PyHamt_Assoc(self, key, val);
+}
+
+static PyObject *
+hamt_py_get(PyHamtObject *self, PyObject *args)
+{
+    PyObject *key;
+    PyObject *def = NULL;
+
+    if (!PyArg_UnpackTuple(args, "get", 1, 2, &key, &def)) {
+        return NULL;
+    }
+
+    PyObject *val = NULL;
+    hamt_find_t res = hamt_find(self, key, &val);
+    switch (res) {
+        case F_ERROR:
+            return NULL;
+        case F_FOUND:
+            Py_INCREF(val);
+            return val;
+        case F_NOT_FOUND:
+            if (def == NULL) {
+                Py_RETURN_NONE;
+            }
+            Py_INCREF(def);
+            return def;
+        default:
+            Py_UNREACHABLE();
+    }
+}
+
+static PyObject *
+hamt_py_delete(PyHamtObject *self, PyObject *key)
+{
+    return (PyObject *)_PyHamt_Without(self, key);
+}
+
+static PyObject *
+hamt_py_items(PyHamtObject *self, PyObject *args)
+{
+    return _PyHamt_NewIterItems(self);
+}
+
+static PyObject *
+hamt_py_values(PyHamtObject *self, PyObject *args)
+{
+    return _PyHamt_NewIterValues(self);
+}
+
+static PyObject *
+hamt_py_keys(PyHamtObject *self, PyObject *args)
+{
+    return _PyHamt_NewIterKeys(self);
+}
+
+#ifdef Py_DEBUG
+static PyObject *
+hamt_py_dump(PyHamtObject *self, PyObject *args)
+{
+    return hamt_dump(self);
+}
+#endif
+
+
+static PyMethodDef PyHamt_methods[] = {
+    {"set", (PyCFunction)hamt_py_set, METH_VARARGS, NULL},
+    {"get", (PyCFunction)hamt_py_get, METH_VARARGS, NULL},
+    {"delete", (PyCFunction)hamt_py_delete, METH_O, NULL},
+    {"items", (PyCFunction)hamt_py_items, METH_NOARGS, NULL},
+    {"keys", (PyCFunction)hamt_py_keys, METH_NOARGS, NULL},
+    {"values", (PyCFunction)hamt_py_values, METH_NOARGS, NULL},
+#ifdef Py_DEBUG
+    {"__dump__", (PyCFunction)hamt_py_dump, METH_NOARGS, NULL},
+#endif
+    {NULL, NULL}
+};
+
+static PySequenceMethods PyHamt_as_sequence = {
+    0,                                /* sq_length */
+    0,                                /* sq_concat */
+    0,                                /* sq_repeat */
+    0,                                /* sq_item */
+    0,                                /* sq_slice */
+    0,                                /* sq_ass_item */
+    0,                                /* sq_ass_slice */
+    (objobjproc)hamt_tp_contains,     /* sq_contains */
+    0,                                /* sq_inplace_concat */
+    0,                                /* sq_inplace_repeat */
+};
+
+static PyMappingMethods PyHamt_as_mapping = {
+    (lenfunc)hamt_tp_len,             /* mp_length */
+    (binaryfunc)hamt_tp_subscript,    /* mp_subscript */
+};
+
+PyTypeObject _PyHamt_Type = {
+    PyVarObject_HEAD_INIT(&PyType_Type, 0)
+    "hamt",
+    sizeof(PyHamtObject),
+    .tp_methods = PyHamt_methods,
+    .tp_as_mapping = &PyHamt_as_mapping,
+    .tp_as_sequence = &PyHamt_as_sequence,
+    .tp_iter = (getiterfunc)hamt_tp_iter,
+    .tp_dealloc = (destructor)hamt_tp_dealloc,
+    .tp_getattro = PyObject_GenericGetAttr,
+    .tp_flags = Py_TPFLAGS_DEFAULT | Py_TPFLAGS_HAVE_GC,
+    .tp_richcompare = hamt_tp_richcompare,
+    .tp_traverse = (traverseproc)hamt_tp_traverse,
+    .tp_clear = (inquiry)hamt_tp_clear,
+    .tp_new = hamt_tp_new,
+    .tp_weaklistoffset = offsetof(PyHamtObject, h_weakreflist),
+    .tp_hash = PyObject_HashNotImplemented,
+};
+
+
+/////////////////////////////////// Tree Node Types
+
+
+PyTypeObject _PyHamt_ArrayNode_Type = {
+    PyVarObject_HEAD_INIT(&PyType_Type, 0)
+    "hamt_array_node",
+    sizeof(PyHamtNode_Array),
+    0,
+    .tp_dealloc = (destructor)hamt_node_array_dealloc,
+    .tp_getattro = PyObject_GenericGetAttr,
+    .tp_flags = Py_TPFLAGS_DEFAULT | Py_TPFLAGS_HAVE_GC,
+    .tp_traverse = (traverseproc)hamt_node_array_traverse,
+    .tp_free = PyObject_GC_Del,
+    .tp_hash = PyObject_HashNotImplemented,
+};
+
+PyTypeObject _PyHamt_BitmapNode_Type = {
+    PyVarObject_HEAD_INIT(&PyType_Type, 0)
+    "hamt_bitmap_node",
+    sizeof(PyHamtNode_Bitmap) - sizeof(PyObject *),
+    sizeof(PyObject *),
+    .tp_dealloc = (destructor)hamt_node_bitmap_dealloc,
+    .tp_getattro = PyObject_GenericGetAttr,
+    .tp_flags = Py_TPFLAGS_DEFAULT | Py_TPFLAGS_HAVE_GC,
+    .tp_traverse = (traverseproc)hamt_node_bitmap_traverse,
+    .tp_free = PyObject_GC_Del,
+    .tp_hash = PyObject_HashNotImplemented,
+};
+
+PyTypeObject _PyHamt_CollisionNode_Type = {
+    PyVarObject_HEAD_INIT(&PyType_Type, 0)
+    "hamt_collision_node",
+    sizeof(PyHamtNode_Collision) - sizeof(PyObject *),
+    sizeof(PyObject *),
+    .tp_dealloc = (destructor)hamt_node_collision_dealloc,
+    .tp_getattro = PyObject_GenericGetAttr,
+    .tp_flags = Py_TPFLAGS_DEFAULT | Py_TPFLAGS_HAVE_GC,
+    .tp_traverse = (traverseproc)hamt_node_collision_traverse,
+    .tp_free = PyObject_GC_Del,
+    .tp_hash = PyObject_HashNotImplemented,
+};
+
+
+int
+_PyHamt_Init(void)
+{
+    if ((PyType_Ready(&_PyHamt_Type) < 0) ||
+        (PyType_Ready(&_PyHamt_ArrayNode_Type) < 0) ||
+        (PyType_Ready(&_PyHamt_BitmapNode_Type) < 0) ||
+        (PyType_Ready(&_PyHamt_CollisionNode_Type) < 0) ||
+        (PyType_Ready(&_PyHamtKeys_Type) < 0) ||
+        (PyType_Ready(&_PyHamtValues_Type) < 0) ||
+        (PyType_Ready(&_PyHamtItems_Type) < 0))
+    {
+        return 0;
+    }
+
+    return 1;
+}
+
+void
+_PyHamt_Fini(void)
+{
+    Py_CLEAR(_empty_hamt);
+    Py_CLEAR(_empty_bitmap_node);
+}
diff --git a/Python/pylifecycle.c b/Python/pylifecycle.c
index 2f61db0d82..d46784a2f6 100644
--- a/Python/pylifecycle.c
+++ b/Python/pylifecycle.c
@@ -4,6 +4,8 @@
 
 #include "Python-ast.h"
 #undef Yield /* undefine macro conflicting with winbase.h */
+#include "internal/context.h"
+#include "internal/hamt.h"
 #include "internal/pystate.h"
 #include "grammar.h"
 #include "node.h"
@@ -758,6 +760,9 @@ _Py_InitializeCore(const _PyCoreConfig *core_config)
         return _Py_INIT_ERR("can't initialize warnings");
     }
 
+    if (!_PyContext_Init())
+        return _Py_INIT_ERR("can't init context");
+
     /* This call sets up builtin and frozen import support */
     if (!interp->core_config._disable_importlib) {
         err = initimport(interp, sysmod);
@@ -1176,6 +1181,7 @@ Py_FinalizeEx(void)
     _Py_HashRandomization_Fini();
     _PyArg_Fini();
     PyAsyncGen_Fini();
+    _PyContext_Fini();
 
     /* Cleanup Unicode implementation */
     _PyUnicode_Fini();
diff --git a/Python/pystate.c b/Python/pystate.c
index 9c25a26460..909d831465 100644
--- a/Python/pystate.c
+++ b/Python/pystate.c
@@ -173,6 +173,8 @@ PyInterpreterState_New(void)
     }
     HEAD_UNLOCK();
 
+    interp->tstate_next_unique_id = 0;
+
     return interp;
 }
 
@@ -313,6 +315,11 @@ new_threadstate(PyInterpreterState *interp, int init)
         tstate->async_gen_firstiter = NULL;
         tstate->async_gen_finalizer = NULL;
 
+        tstate->context = NULL;
+        tstate->context_ver = 1;
+
+        tstate->id = ++interp->tstate_next_unique_id;
+
         if (init)
             _PyThreadState_Init(tstate);
 
@@ -499,6 +506,8 @@ PyThreadState_Clear(PyThreadState *tstate)
     Py_CLEAR(tstate->coroutine_wrapper);
     Py_CLEAR(tstate->async_gen_firstiter);
     Py_CLEAR(tstate->async_gen_finalizer);
+
+    Py_CLEAR(tstate->context);
 }
 
 
diff --git a/Tools/msi/lib/lib_files.wxs b/Tools/msi/lib/lib_files.wxs
index 5a72612c6a..46ddcb41e9 100644
--- a/Tools/msi/lib/lib_files.wxs
+++ b/Tools/msi/lib/lib_files.wxs
@@ -1,6 +1,6 @@
 <?xml version="1.0" encoding="UTF-8"?>
 <Wix xmlns="http://schemas.microsoft.com/wix/2006/wi">
-    <?define exts=pyexpat;select;unicodedata;winsound;_bz2;_elementtree;_socket;_ssl;_msi;_ctypes;_hashlib;_multiprocessing;_lzma;_decimal;_overlapped;_sqlite3;_asyncio;_queue;_distutils_findvs ?>
+    <?define exts=pyexpat;select;unicodedata;winsound;_bz2;_elementtree;_socket;_ssl;_msi;_ctypes;_hashlib;_multiprocessing;_lzma;_decimal;_overlapped;_sqlite3;_asyncio;_queue;_distutils_findvs;_contextvars ?>
     <Fragment>
         <ComponentGroup Id="lib_extensions">
             <?foreach ext in $(var.exts)?>
diff --git a/setup.py b/setup.py
index 1da40a426a..258094e3ad 100644
--- a/setup.py
+++ b/setup.py
@@ -644,6 +644,9 @@ class PyBuildExt(build_ext):
         # array objects
         exts.append( Extension('array', ['arraymodule.c']) )
 
+        # Context Variables
+        exts.append( Extension('_contextvars', ['_contextvarsmodule.c']) )
+
         shared_math = 'Modules/_math.o'
         # complex math library functions
         exts.append( Extension('cmath', ['cmathmodule.c'],