bpo-39481: Implementation for PEP 585 (#18239)

This implements things like `list[int]`,
which returns an object of type `types.GenericAlias`.
This object mostly acts as a proxy for `list`,
but has attributes `__origin__` and `__args__`
that allow recovering the parts (with values `list` and `(int,)`.

There is also an approximate notion of type variables;
e.g. `list[T]` has a `__parameters__` attribute equal to `(T,)`.
Type variables are objects of type `typing.TypeVar`.

1 files changed, 3 insertions, 21 deletions

diff --git a/Lib/subprocess.py b/Lib/subprocess.py
index 1eecceaed4..86fdf27f9b 100644
--- a/Lib/subprocess.py
+++ b/Lib/subprocess.py
@@ -52,6 +52,7 @@ import threading
 import warnings
 import contextlib
 from time import monotonic as _time
+import types
 
 try:
     import pwd
@@ -446,17 +447,7 @@ class CompletedProcess(object):
             args.append('stderr={!r}'.format(self.stderr))
         return "{}({})".format(type(self).__name__, ', '.join(args))
 
-    def __class_getitem__(cls, type):
-        """Provide minimal support for using this class as generic
-        (for example in type annotations).
-
-        See PEP 484 and PEP 560 for more details. For example,
-        `CompletedProcess[bytes]` is a valid expression at runtime
-        (type argument `bytes` indicates the type used for stdout).
-        Note, no type checking happens at runtime, but a static type
-        checker can be used.
-        """
-        return cls
+    __class_getitem__ = classmethod(types.GenericAlias)
 
 
     def check_returncode(self):
@@ -1000,16 +991,7 @@ class Popen(object):
             obj_repr = obj_repr[:76] + "...>"
         return obj_repr
 
-    def __class_getitem__(cls, type):
-        """Provide minimal support for using this class as generic
-        (for example in type annotations).
-
-        See PEP 484 and PEP 560 for more details. For example, `Popen[bytes]`
-        is a valid expression at runtime (type argument `bytes` indicates the
-        type used for stdout). Note, no type checking happens at runtime, but
-        a static type checker can be used.
-        """
-        return cls
+    __class_getitem__ = classmethod(types.GenericAlias)
 
     @property
     def universal_newlines(self):