Backport parameter defaults for (Async)Generator and (Async)ContextManager (#382)

Author: AlexWaygood

CHANGELOG.md

@@ -22,6 +22,15 @@
 - At runtime, `assert_never` now includes the repr of the argument
   in the `AssertionError`. Patch by Hashem, backporting of the original
   fix https://github.com/python/cpython/pull/91720 by Jelle Zijlstra.
+- The second and third parameters of `typing_extensions.Generator`,
+  and the second parameter of `typing_extensions.AsyncGenerator`,
+  now default to `None`. This matches the behaviour of `typing.Generator`
+  and `typing.AsyncGenerator` on Python 3.13+.
+- `typing.ContextManager` and `typing.AsyncContextManager` now have an
+  optional second parameter, which defaults to `Optional[bool]`. The new
+  parameter signifies the return type of the `__(a)exit__` method,
+  matching `typing.ContextManager` and `typing.AsyncContextManager` on
+  Python 3.13+.
 
 # Release 4.11.0 (April 5, 2024)
 

doc/index.rst

@@ -885,8 +885,8 @@ Annotation metadata
 Pure aliases
 ~~~~~~~~~~~~
 
-These are simply re-exported from the :mod:`typing` module on all supported
-versions of Python. They are listed here for completeness.
+Most of these are simply re-exported from the :mod:`typing` module on all supported
+versions of Python, but all are listed here for completeness.
 
 .. class:: AbstractSet
 
@@ -904,10 +904,19 @@ versions of Python. They are listed here for completeness.
 
    See :py:class:`typing.AsyncContextManager`. In ``typing`` since 3.5.4 and 3.6.2.
 
+   .. versionchanged:: 4.12.0
+
+      ``AsyncContextManager`` now has an optional second parameter, defaulting to
+      ``Optional[bool]``, signifying the return type of the ``__aexit__`` method.
+
 .. class:: AsyncGenerator
 
    See :py:class:`typing.AsyncGenerator`. In ``typing`` since 3.6.1.
 
+   .. versionchanged:: 4.12.0
+
+      The second type parameter is now optional (it defaults to ``None``).
+
 .. class:: AsyncIterable
 
    See :py:class:`typing.AsyncIterable`. In ``typing`` since 3.5.2.
@@ -956,6 +965,11 @@ versions of Python. They are listed here for completeness.
 
    See :py:class:`typing.ContextManager`. In ``typing`` since 3.5.4.
 
+   .. versionchanged:: 4.12.0
+
+      ``AsyncContextManager`` now has an optional second parameter, defaulting to
+      ``Optional[bool]``, signifying the return type of the ``__aexit__`` method.
+
 .. class:: Coroutine
 
    See :py:class:`typing.Coroutine`. In ``typing`` since 3.5.3.
@@ -996,6 +1010,11 @@ versions of Python. They are listed here for completeness.
 
    .. versionadded:: 4.7.0
 
+   .. versionchanged:: 4.12.0
+
+      The second type and third type parameters are now optional
+      (they both default to ``None``).
+
 .. class:: Generic
 
    See :py:class:`typing.Generic`.

src/test_typing_extensions.py

@@ -41,6 +41,8 @@
 from typing_extensions import Doc, NoDefault
 from _typed_dict_test_helper import Foo, FooGeneric, VeryAnnotated
 
+NoneType = type(None)
+
 # Flags used to mark tests that only apply after a specific
 # version of the typing module.
 TYPING_3_9_0 = sys.version_info[:3] >= (3, 9, 0)
@@ -1626,6 +1628,17 @@ async def g(): yield 0
         self.assertNotIsInstance(type(g), G)
         self.assertNotIsInstance(g, G)
 
+    def test_generator_default(self):
+        g1 = typing_extensions.Generator[int]
+        g2 = typing_extensions.Generator[int, None, None]
+        self.assertEqual(get_args(g1), (int, type(None), type(None)))
+        self.assertEqual(get_args(g1), get_args(g2))
+
+        g3 = typing_extensions.Generator[int, float]
+        g4 = typing_extensions.Generator[int, float, None]
+        self.assertEqual(get_args(g3), (int, float, type(None)))
+        self.assertEqual(get_args(g3), get_args(g4))
+
 
 class OtherABCTests(BaseTestCase):
 
@@ -1638,6 +1651,12 @@ def manager():
         self.assertIsInstance(cm, typing_extensions.ContextManager)
         self.assertNotIsInstance(42, typing_extensions.ContextManager)
 
+    def test_contextmanager_type_params(self):
+        cm1 = typing_extensions.ContextManager[int]
+        self.assertEqual(get_args(cm1), (int, typing.Optional[bool]))
+        cm2 = typing_extensions.ContextManager[int, None]
+        self.assertEqual(get_args(cm2), (int, NoneType))
+
     def test_async_contextmanager(self):
         class NotACM:
             pass
@@ -1649,11 +1668,20 @@ def manager():
 
         cm = manager()
         self.assertNotIsInstance(cm, typing_extensions.AsyncContextManager)
-        self.assertEqual(typing_extensions.AsyncContextManager[int].__args__, (int,))
+        self.assertEqual(
+            typing_extensions.AsyncContextManager[int].__args__,
+            (int, typing.Optional[bool])
+        )
         with self.assertRaises(TypeError):
             isinstance(42, typing_extensions.AsyncContextManager[int])
         with self.assertRaises(TypeError):
-            typing_extensions.AsyncContextManager[int, str]
+            typing_extensions.AsyncContextManager[int, str, float]
+
+    def test_asynccontextmanager_type_params(self):
+        cm1 = typing_extensions.AsyncContextManager[int]
+        self.assertEqual(get_args(cm1), (int, typing.Optional[bool]))
+        cm2 = typing_extensions.AsyncContextManager[int, None]
+        self.assertEqual(get_args(cm2), (int, NoneType))
 
 
 class TypeTests(BaseTestCase):
@@ -5533,28 +5561,25 @@ def test_all_names_in___all__(self):
         self.assertLessEqual(exclude, actual_names)
 
     def test_typing_extensions_defers_when_possible(self):
-        exclude = {
-            'dataclass_transform',
-            'overload',
-            'ParamSpec',
-            'TypeVar',
-            'TypeVarTuple',
-            'get_type_hints',
-        }
+        exclude = set()
         if sys.version_info < (3, 10):
             exclude |= {'get_args', 'get_origin'}
         if sys.version_info < (3, 10, 1):
             exclude |= {"Literal"}
         if sys.version_info < (3, 11):
-            exclude |= {'final', 'Any', 'NewType'}
+            exclude |= {'final', 'Any', 'NewType', 'overload'}
         if sys.version_info < (3, 12):
             exclude |= {
                 'SupportsAbs', 'SupportsBytes',
                 'SupportsComplex', 'SupportsFloat', 'SupportsIndex', 'SupportsInt',
-                'SupportsRound', 'Unpack',
+                'SupportsRound', 'Unpack', 'dataclass_transform',
             }
         if sys.version_info < (3, 13):
-            exclude |= {'NamedTuple', 'Protocol', 'runtime_checkable'}
+            exclude |= {
+                'NamedTuple', 'Protocol', 'runtime_checkable', 'Generator',
+                'AsyncGenerator', 'ContextManager', 'AsyncContextManager',
+                'ParamSpec', 'TypeVar', 'TypeVarTuple', 'get_type_hints',
+            }
         if not typing_extensions._PEP_728_IMPLEMENTED:
             exclude |= {'TypedDict', 'is_typeddict'}
         for item in typing_extensions.__all__:

src/typing_extensions.py

@@ -1,6 +1,7 @@
 import abc
 import collections
 import collections.abc
+import contextlib
 import functools
 import inspect
 import operator
@@ -408,17 +409,96 @@ def clear_overloads():
 AsyncIterable = typing.AsyncIterable
 AsyncIterator = typing.AsyncIterator
 Deque = typing.Deque
-ContextManager = typing.ContextManager
-AsyncContextManager = typing.AsyncContextManager
 DefaultDict = typing.DefaultDict
 OrderedDict = typing.OrderedDict
 Counter = typing.Counter
 ChainMap = typing.ChainMap
-AsyncGenerator = typing.AsyncGenerator
 Text = typing.Text
 TYPE_CHECKING = typing.TYPE_CHECKING
 
 
+if sys.version_info >= (3, 13, 0, "beta"):
+    from typing import ContextManager, AsyncContextManager, Generator, AsyncGenerator
+else:
+    def _is_dunder(attr):
+        return attr.startswith('__') and attr.endswith('__')
+
+    # Python <3.9 doesn't have typing._SpecialGenericAlias
+    _special_generic_alias_base = getattr(
+        typing, "_SpecialGenericAlias", typing._GenericAlias
+    )
+
+    class _SpecialGenericAlias(_special_generic_alias_base, _root=True):
+        def __init__(self, origin, nparams, *, inst=True, name=None, defaults=()):
+            if _special_generic_alias_base is typing._GenericAlias:
+                # Python <3.9
+                self.__origin__ = origin
+                self._nparams = nparams
+                super().__init__(origin, nparams, special=True, inst=inst, name=name)
+            else:
+                # Python >= 3.9
+                super().__init__(origin, nparams, inst=inst, name=name)
+            self._defaults = defaults
+
+        def __setattr__(self, attr, val):
+            allowed_attrs = {'_name', '_inst', '_nparams', '_defaults'}
+            if _special_generic_alias_base is typing._GenericAlias:
+                # Python <3.9
+                allowed_attrs.add("__origin__")
+            if _is_dunder(attr) or attr in allowed_attrs:
+                object.__setattr__(self, attr, val)
+            else:
+                setattr(self.__origin__, attr, val)
+
+        @typing._tp_cache
+        def __getitem__(self, params):
+            if not isinstance(params, tuple):
+                params = (params,)
+            msg = "Parameters to generic types must be types."
+            params = tuple(typing._type_check(p, msg) for p in params)
+            if (
+                self._defaults
+                and len(params) < self._nparams
+                and len(params) + len(self._defaults) >= self._nparams
+            ):
+                params = (*params, *self._defaults[len(params) - self._nparams:])
+            actual_len = len(params)
+
+            if actual_len != self._nparams:
+                if self._defaults:
+                    expected = f"at least {self._nparams - len(self._defaults)}"
+                else:
+                    expected = str(self._nparams)
+                if not self._nparams:
+                    raise TypeError(f"{self} is not a generic class")
+                raise TypeError(
+                    f"Too {'many' if actual_len > self._nparams else 'few'}"
+                    f" arguments for {self};"
+                    f" actual {actual_len}, expected {expected}"
+                )
+            return self.copy_with(params)
+
+    _NoneType = type(None)
+    Generator = _SpecialGenericAlias(
+        collections.abc.Generator, 3, defaults=(_NoneType, _NoneType)
+    )
+    AsyncGenerator = _SpecialGenericAlias(
+        collections.abc.AsyncGenerator, 2, defaults=(_NoneType,)
+    )
+    ContextManager = _SpecialGenericAlias(
+        contextlib.AbstractContextManager,
+        2,
+        name="ContextManager",
+        defaults=(typing.Optional[bool],)
+    )
+    AsyncContextManager = _SpecialGenericAlias(
+        contextlib.AbstractAsyncContextManager,
+        2,
+        name="AsyncContextManager",
+        defaults=(typing.Optional[bool],)
+    )
+
+
 _PROTO_ALLOWLIST = {
     'collections.abc': [
         'Callable', 'Awaitable', 'Iterable', 'Iterator', 'AsyncIterable',
@@ -3344,7 +3424,6 @@ def __eq__(self, other: object) -> bool:
 Dict = typing.Dict
 ForwardRef = typing.ForwardRef
 FrozenSet = typing.FrozenSet
-Generator = typing.Generator
 Generic = typing.Generic
 Hashable = typing.Hashable
 IO = typing.IO