Enhance representation of Future and Future subclasses

* Add "created at filename:lineno" in the representation
* Add Future._repr_info() method which can be more easily overriden than
  Future.__repr__(). It should now be more easy to enhance Future
  representation without having to modify each subclass. For example,
  _OverlappedFuture and _WaitHandleFuture get the new "created at" information.
* Use reprlib to format Future result, and function arguments when formatting a
  callback, to limit the length of the representation.

6 files changed, 60 insertions, 52 deletions

diff --git a/asyncio/events.py b/asyncio/events.py
index bddd7e3..3c7a36d 100644
--- a/asyncio/events.py
+++ b/asyncio/events.py
@@ -10,11 +10,12 @@ __all__ = ['AbstractEventLoopPolicy',
 
 import functools
 import inspect
-import subprocess
-import traceback
-import threading
+import reprlib
 import socket
+import subprocess
 import sys
+import threading
+import traceback
 
 
 _PY34 = sys.version_info >= (3, 4)
@@ -36,8 +37,12 @@ def _get_function_source(func):
 
 
 def _format_args(args):
-    # function formatting ('hello',) as ('hello')
-    args_repr = repr(args)
+    """Format function arguments.
+
+    Special case for a single parameter: ('hello',) is formatted as ('hello').
+    """
+    # use reprlib to limit the length of the output
+    args_repr = reprlib.repr(args)
     if len(args) == 1 and args_repr.endswith(',)'):
         args_repr = args_repr[:-2] + ')'
     return args_repr
diff --git a/asyncio/futures.py b/asyncio/futures.py
index 022fef7..7998fbb 100644
--- a/asyncio/futures.py
+++ b/asyncio/futures.py
@@ -7,6 +7,7 @@ __all__ = ['CancelledError', 'TimeoutError',
 
 import concurrent.futures._base
 import logging
+import reprlib
 import sys
 import traceback
 
@@ -175,20 +176,25 @@ class Future:
                                             format_cb(cb[-1]))
         return 'cb=[%s]' % cb
 
-    def _format_result(self):
-        if self._state != _FINISHED:
-            return None
-        elif self._exception is not None:
-            return 'exception={!r}'.format(self._exception)
-        else:
-            return 'result={!r}'.format(self._result)
-
-    def __repr__(self):
+    def _repr_info(self):
         info = [self._state.lower()]
         if self._state == _FINISHED:
-            info.append(self._format_result())
+            if self._exception is not None:
+                info.append('exception={!r}'.format(self._exception))
+            else:
+                # use reprlib to limit the length of the output, especially
+                # for very long strings
+                result = reprlib.repr(self._result)
+                info.append('result={}'.format(result))
         if self._callbacks:
             info.append(self._format_callbacks())
+        if self._source_traceback:
+            frame = self._source_traceback[-1]
+            info.append('created at %s:%s' % (frame[0], frame[1]))
+        return info
+
+    def __repr__(self):
+        info = self._repr_info()
         return '<%s %s>' % (self.__class__.__name__, ' '.join(info))
 
     # On Python 3.3 or older, objects with a destructor part of a reference
diff --git a/asyncio/tasks.py b/asyncio/tasks.py
index 07952c9..9207016 100644
--- a/asyncio/tasks.py
+++ b/asyncio/tasks.py
@@ -92,30 +92,19 @@ class Task(futures.Future):
                 self._loop.call_exception_handler(context)
             futures.Future.__del__(self)
 
-    def __repr__(self):
-        info = []
+    def _repr_info(self):
+        info = super()._repr_info()
+
         if self._must_cancel:
-            info.append('cancelling')
-        else:
-            info.append(self._state.lower())
+            # replace status
+            info[0] = 'cancelling'
 
         coro = coroutines._format_coroutine(self._coro)
-        info.append('coro=<%s>' % coro)
-
-        if self._source_traceback:
-            frame = self._source_traceback[-1]
-            info.append('created at %s:%s' % (frame[0], frame[1]))
-
-        if self._state == futures._FINISHED:
-            info.append(self._format_result())
-
-        if self._callbacks:
-            info.append(self._format_callbacks())
+        info.insert(1, 'coro=<%s>' % coro)
 
         if self._fut_waiter is not None:
-            info.append('wait_for=%r' % self._fut_waiter)
-
-        return '<%s %s>' % (self.__class__.__name__, ' '.join(info))
+            info.insert(2, 'wait_for=%r' % self._fut_waiter)
+        return info
 
     def get_stack(self, *, limit=None):
         """Return the list of stack frames for this task's coroutine.
diff --git a/asyncio/windows_events.py b/asyncio/windows_events.py
index 6d9feab..1db255e 100644
--- a/asyncio/windows_events.py
+++ b/asyncio/windows_events.py
@@ -42,16 +42,12 @@ class _OverlappedFuture(futures.Future):
             del self._source_traceback[-1]
         self._ov = ov
 
-    def __repr__(self):
-        info = [self._state.lower()]
+    def _repr_info(self):
+        info = super()._repr_info()
         if self._ov is not None:
             state = 'pending' if self._ov.pending else 'completed'
-            info.append('overlapped=<%s, %#x>' % (state, self._ov.address))
-        if self._state == futures._FINISHED:
-            info.append(self._format_result())
-        if self._callbacks:
-            info.append(self._format_callbacks())
-        return '<%s %s>' % (self.__class__.__name__, ' '.join(info))
+            info.insert(1, 'overlapped=<%s, %#x>' % (state, self._ov.address))
+        return info
 
     def _cancel_overlapped(self):
         if self._ov is None:
@@ -95,17 +91,14 @@ class _WaitHandleFuture(futures.Future):
         return (_winapi.WaitForSingleObject(self._handle, 0) ==
                 _winapi.WAIT_OBJECT_0)
 
-    def __repr__(self):
-        info = [self._state.lower()]
+    def _repr_info(self):
+        info = super()._repr_info()
+        info.insert(1, 'handle=%#x' % self._handle)
         if self._wait_handle:
             state = 'pending' if self._poll() else 'completed'
-            info.append('wait_handle=<%s, %#x>' % (state, self._wait_handle))
-        info.append('handle=<%#x>' % self._handle)
-        if self._state == futures._FINISHED:
-            info.append(self._format_result())
-        if self._callbacks:
-            info.append(self._format_callbacks())
-        return '<%s %s>' % (self.__class__.__name__, ' '.join(info))
+            info.insert(1, 'wait_handle=<%s, %#x>'
+                           % (state, self._wait_handle))
+        return info
 
     def _unregister(self):
         if self._wait_handle is None:
diff --git a/tests/test_futures.py b/tests/test_futures.py
index 50e9414..3029a9c 100644
--- a/tests/test_futures.py
+++ b/tests/test_futures.py
@@ -105,6 +105,15 @@ class FutureTests(test_utils.TestCase):
         self.assertEqual(next(g), ('C', 42))  # yield 'C', y.
 
     def test_future_repr(self):
+        self.loop.set_debug(True)
+        f_pending_debug = asyncio.Future(loop=self.loop)
+        frame = f_pending_debug._source_traceback[-1]
+        self.assertEqual(repr(f_pending_debug),
+                         '<Future pending created at %s:%s>'
+                         % (frame[0], frame[1]))
+        f_pending_debug.cancel()
+
+        self.loop.set_debug(False)
         f_pending = asyncio.Future(loop=self.loop)
         self.assertEqual(repr(f_pending), '<Future pending>')
         f_pending.cancel()
@@ -324,7 +333,7 @@ class FutureTests(test_utils.TestCase):
         if sys.version_info >= (3, 4):
             frame = source_traceback[-1]
             regex = (r'^Future exception was never retrieved\n'
-                     r'future: <Future finished exception=MemoryError\(\)>\n'
+                     r'future: <Future finished exception=MemoryError\(\) created at {filename}:{lineno}>\n'
                      r'source_traceback: Object created at \(most recent call last\):\n'
                      r'  File'
                      r'.*\n'
diff --git a/tests/test_tasks.py b/tests/test_tasks.py
index 7b93a0e..95cba54 100644
--- a/tests/test_tasks.py
+++ b/tests/test_tasks.py
@@ -132,6 +132,8 @@ class TaskTests(test_utils.TestCase):
             asyncio.async('ok')
 
     def test_task_repr(self):
+        self.loop.set_debug(False)
+
         @asyncio.coroutine
         def notmuch():
             yield from []
@@ -189,6 +191,8 @@ class TaskTests(test_utils.TestCase):
                          "<Task finished %s result='abc'>" % coro)
 
     def test_task_repr_coro_decorator(self):
+        self.loop.set_debug(False)
+
         @asyncio.coroutine
         def notmuch():
             # notmuch() function doesn't use yield from: it will be wrapped by
@@ -252,6 +256,8 @@ class TaskTests(test_utils.TestCase):
         self.loop.run_until_complete(t)
 
     def test_task_repr_wait_for(self):
+        self.loop.set_debug(False)
+
         @asyncio.coroutine
         def wait_for(fut):
             return (yield from fut)