add typing.ContextManager on all supported Python versions

So that there is a way to type hint for a context manager even on language versions before 3.6. This is just copied from contextlib.AbstractContextManager in 3.6. Fixes #274
python · gvanrossum · May 6, 2017 · May 3, 2017 · May 3, 2017 · May 4, 2017
commit bf8cb7ea6f26ee4d7286ababd9c4016f108f4046
diff --git a/python2/test_typing.py b/python2/test_typing.py
@@ -1,6 +1,7 @@
 from __future__ import absolute_import, unicode_literals
 
 import collections
+import contextlib
 import pickle
    import re
 import sys
@@ -1653,6 +1654,18 @@ class B: pass
         self.assertIsSubclass(B, typing.Mapping)
 
 
+class OtherABCTests(BaseTestCase):
+
+    def test_contextmanager(self):
+        @contextlib.contextmanager
+        def manager():
+            yield 42
+
+        cm = manager()
+        self.assertIsInstance(cm, typing.ContextManager)
+        self.assertNotIsInstance(42, typing.ContextManager)
+
+
 class TypeTests(BaseTestCase):
 
     def test_type_basic(self):

diff --git a/python2/typing.py b/python2/typing.py
@@ -32,6 +32,7 @@
                     # for 'Generic' and ABCs below.
     'ByteString',
     'Container',
+    'ContextManager',
     'Hashable',
     'ItemsView',
     'Iterable',
@@ -1857,6 +1858,26 @@ class ValuesView(MappingView[VT_co]):
     __extra__ = collections_abc.ValuesView
 
 
+class ContextManager(Generic[T_co]):
+    __slots__ = ()
+
+    def __enter__(self):
+        return self
+
+    @abc.abstractmethod
+    def __exit__(self, exc_type, exc_value, traceback):
+        """Raise any exception triggered within the runtime context."""
+        return None
+
+    @classmethod
+    def __subclasshook__(cls, C):
+        if cls is ContextManager:
+            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
+
+
 class Dict(dict, MutableMapping[KT, VT]):
     __slots__ = ()
     __extra__ = dict

diff --git a/src/test_typing.py b/src/test_typing.py
@@ -2156,8 +2156,6 @@ class B: ...
 
 class OtherABCTests(BaseTestCase):
 
-    @skipUnless(hasattr(typing, 'ContextManager'),
-                'requires typing.ContextManager')
     def test_contextmanager(self):
         @contextlib.contextmanager
         def manager():

diff --git a/src/typing.py b/src/typing.py
@@ -37,6 +37,7 @@
                     # for 'Generic' and ABCs below.
     'ByteString',
     'Container',
+    'ContextManager',
     'Hashable',
     'ItemsView',
     'Iterable',
@@ -57,7 +58,6 @@
     # AsyncIterable,
     # Coroutine,
     # Collection,
-    # ContextManager,
     # AsyncGenerator,
 
     # Structural checks, a.k.a. protocols.
@@ -1949,7 +1949,25 @@ 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):
+            """Raise any exception triggered within the runtime context."""
+            return None
+
+        @classmethod
+        def __subclasshook__(cls, C):
+            if cls is ContextManager:
+                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
 
 
 class Dict(dict, MutableMapping[KT, VT], extra=dict):