bpo-28556: Updates to typing module (#2076)

This PR contains two updates to typing module:

- Support ContextManager on all versions (original PR by Jelle Zijlstra).
- Add generic AsyncContextManager.

3 files changed, 99 insertions, 5 deletions

diff --git a/Lib/test/test_typing.py b/Lib/test/test_typing.py
index 33d553e..fd2d93c 100644
--- a/Lib/test/test_typing.py
+++ b/Lib/test/test_typing.py
@@ -1552,6 +1552,12 @@ class AsyncIteratorWrapper(typing.AsyncIterator[T_a]):
             return data
         else:
             raise StopAsyncIteration
+
+class ACM:
+    async def __aenter__(self) -> int:
+        return 42
+    async def __aexit__(self, etype, eval, tb):
+        return None
 """
 
 if ASYNCIO:
@@ -1562,12 +1568,13 @@ if ASYNCIO:
 else:
     # fake names for the sake of static analysis
     asyncio = None
-    AwaitableWrapper = AsyncIteratorWrapper = object
+    AwaitableWrapper = AsyncIteratorWrapper = ACM = object
 
 PY36 = sys.version_info[:2] >= (3, 6)
 
 PY36_TESTS = """
 from test import ann_module, ann_module2, ann_module3
+from typing import AsyncContextManager
 
 class A:
     y: float
@@ -1604,6 +1611,16 @@ class XRepr(NamedTuple):
         return f'{self.x} -> {self.y}'
     def __add__(self, other):
         return 0
+
+async def g_with(am: AsyncContextManager[int]):
+    x: int
+    async with am as x:
+        return x
+
+try:
+    g_with(ACM()).send(None)
+except StopIteration as e:
+    assert e.args[0] == 42
 """
 
 if PY36:
@@ -2156,8 +2173,6 @@ class CollectionsAbcTests(BaseTestCase):
 
 class OtherABCTests(BaseTestCase):
 
-    @skipUnless(hasattr(typing, 'ContextManager'),
-                'requires typing.ContextManager')
     def test_contextmanager(self):
         @contextlib.contextmanager
         def manager():
@@ -2167,6 +2182,24 @@ class OtherABCTests(BaseTestCase):
         self.assertIsInstance(cm, typing.ContextManager)
         self.assertNotIsInstance(42, typing.ContextManager)
 
+    @skipUnless(ASYNCIO, 'Python 3.5 required')
+    def test_async_contextmanager(self):
+        class NotACM:
+            pass
+        self.assertIsInstance(ACM(), typing.AsyncContextManager)
+        self.assertNotIsInstance(NotACM(), typing.AsyncContextManager)
+        @contextlib.contextmanager
+        def manager():
+            yield 42
+
+        cm = manager()
+        self.assertNotIsInstance(cm, typing.AsyncContextManager)
+        self.assertEqual(typing.AsyncContextManager[int].__args__, (int,))
+        with self.assertRaises(TypeError):
+            isinstance(42, typing.AsyncContextManager[int])
+        with self.assertRaises(TypeError):
+            typing.AsyncContextManager[int, str]
+
 
 class TypeTests(BaseTestCase):
 
diff --git a/Lib/typing.py b/Lib/typing.py
index 645bc6f..c487afc 100644
--- a/Lib/typing.py
+++ b/Lib/typing.py
@@ -10,6 +10,8 @@ try:
     import collections.abc as collections_abc
 except ImportError:
     import collections as collections_abc  # Fallback for PY3.2.
+if sys.version_info[:2] >= (3, 6):
+    import _collections_abc  # Needed for private function _check_methods # noqa
 try:
     from types import WrapperDescriptorType, MethodWrapperType, MethodDescriptorType
 except ImportError:
@@ -37,6 +39,7 @@ __all__ = [
                     # for 'Generic' and ABCs below.
     'ByteString',
     'Container',
+    'ContextManager',
     'Hashable',
     'ItemsView',
     'Iterable',
@@ -57,8 +60,8 @@ __all__ = [
     # AsyncIterable,
     # Coroutine,
     # Collection,
-    # ContextManager,
     # AsyncGenerator,
+    # AsyncContextManager
 
     # Structural checks, a.k.a. protocols.
     'Reversible',
@@ -1949,7 +1952,61 @@ class ValuesView(MappingView[VT_co], extra=collections_abc.ValuesView):
 if hasattr(contextlib, 'AbstractContextManager'):
     class ContextManager(Generic[T_co], extra=contextlib.AbstractContextManager):
         __slots__ = ()
-    __all__.append('ContextManager')
+else:
+    class ContextManager(Generic[T_co]):
+        __slots__ = ()
+
+        def __enter__(self):
+            return self
+
+        @abc.abstractmethod
+        def __exit__(self, exc_type, exc_value, traceback):
+            return None
+
+        @classmethod
+        def __subclasshook__(cls, C):
+            if cls is ContextManager:
+                # In Python 3.6+, it is possible to set a method to None to
+                # explicitly indicate that the class does not implement an ABC
+                # (https://bugs.python.org/issue25958), but we do not support
+                # that pattern here because this fallback class is only used
+                # in Python 3.5 and earlier.
+                if (any("__enter__" in B.__dict__ for B in C.__mro__) and
+                    any("__exit__" in B.__dict__ for B in C.__mro__)):
+                    return True
+            return NotImplemented
+
+
+if hasattr(contextlib, 'AbstractAsyncContextManager'):
+    class AsyncContextManager(Generic[T_co],
+                              extra=contextlib.AbstractAsyncContextManager):
+        __slots__ = ()
+
+    __all__.append('AsyncContextManager')
+elif sys.version_info[:2] >= (3, 5):
+    exec("""
+class AsyncContextManager(Generic[T_co]):
+    __slots__ = ()
+
+    async def __aenter__(self):
+        return self
+
+    @abc.abstractmethod
+    async def __aexit__(self, exc_type, exc_value, traceback):
+        return None
+
+    @classmethod
+    def __subclasshook__(cls, C):
+        if cls is AsyncContextManager:
+            if sys.version_info[:2] >= (3, 6):
+                return _collections_abc._check_methods(C, "__aenter__", "__aexit__")
+            if (any("__aenter__" in B.__dict__ for B in C.__mro__) and
+                    any("__aexit__" in B.__dict__ for B in C.__mro__)):
+                return True
+        return NotImplemented
+
+__all__.append('AsyncContextManager')
+""")
 
 
 class Dict(dict, MutableMapping[KT, VT], extra=dict):
diff --git a/Misc/NEWS b/Misc/NEWS
index 185ec0e..20cea47 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -354,6 +354,10 @@ Library
   non-blocking mode if it succeeded to aquire the lock but the acquire took
   longer than the timeout.
 
+- bpo-28556: Updates to typing module: Add generic AsyncContextManager, add
+  support for ContextManager on all versions. Original PRs by Jelle Zijlstra
+  and Ivan Levkivskyi
+
 - bpo-30605: re.compile() no longer raises a BytesWarning when compiling a
   bytes instance with misplaced inline modifier.  Patch by Roy Williams.