python · 1st1 · Dec 14, 2017 · May 1, 2017 · May 4, 2017 · Oct 11, 2017
diff --git a/Doc/library/contextlib.rst b/Doc/library/contextlib.rst
@@ -29,6 +29,15 @@ Functions and classes provided:
    .. versionadded:: 3.6
 
 
+.. class:: AbstractAsyncContextManager
+
+   An :term:`abstract base class` similar to
+   :class:`~contextlib.AbstractContextManager`, but for
+   :ref:`asynchronous context managers <async-context-managers>`, which
+   implement :meth:`object.__aenter__` and :meth:`object.__aexit__`.
+
+   .. versionadded:: 3.7
+
 
 .. decorator:: contextmanager
 

diff --git a/Doc/whatsnew/3.7.rst b/Doc/whatsnew/3.7.rst
@@ -105,8 +105,9 @@ instead of spaces.  (Contributed by Xiang Zhang in :issue:`30103`.)
 contextlib
 ----------
 
-:func:`contextlib.asynccontextmanager` has been added. (Contributed by
-Jelle Zijlstra in :issue:`29679`.)
+:func:`~contextlib.asynccontextmanager` and
+:class:`~contextlib.AbstractAsyncContextManager` have been added. (Contributed
+by Jelle Zijlstra in :issue:`29679` and :issue:`30241`.)
 
 distutils
 ---------

diff --git a/Lib/contextlib.py b/Lib/contextlib.py
@@ -5,7 +5,8 @@
 from functools import wraps
 
 __all__ = ["asynccontextmanager", "contextmanager", "closing",
-           "AbstractContextManager", "ContextDecorator", "ExitStack",
+           "AbstractContextManager", "AbstractAsyncContextManager",
+           "ContextDecorator", "ExitStack",
            "redirect_stdout", "redirect_stderr", "suppress"]
 
 
@@ -31,6 +32,28 @@ def __subclasshook__(cls, C):
         return NotImplemented
 
 
+class AbstractAsyncContextManager(abc.ABC):
+
+    """An abstract base class for asynchronous context managers."""
+
+    async def __aenter__(self):
+        """Return `self` upon entering the runtime context."""
+        return self
+
+    @abc.abstractmethod
+    async def __aexit__(self, exc_type, exc_value, traceback):
+        """Raise any exception triggered within the runtime context."""
+        return None
+
+    @classmethod
+    def __subclasshook__(cls, C):
+        if cls is AbstractAsyncContextManager:
+            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
+
+
 class ContextDecorator(object):
     "A base class or mixin that enables context managers to work as decorators."
 
@@ -137,7 +160,8 @@ def __exit__(self, type, value, traceback):
             raise RuntimeError("generator didn't stop after throw()")
 
 
-class _AsyncGeneratorContextManager(_GeneratorContextManagerBase):
+class _AsyncGeneratorContextManager(_GeneratorContextManagerBase,
+                                    AbstractAsyncContextManager):
     """Helper for @asynccontextmanager."""
 
     async def __aenter__(self):

diff --git a/Lib/test/test_contextlib_async.py b/Lib/test/test_contextlib_async.py
@@ -1,5 +1,5 @@
 import asyncio
-from contextlib import asynccontextmanager
+from contextlib import asynccontextmanager, AbstractAsyncContextManager
 import functools
 from test import support
 import unittest
@@ -20,6 +20,40 @@ def wrapper(*args, **kwargs):
     return wrapper
 
 
+class TestAbstractAsyncContextManager(unittest.TestCase):
+
+    @_async_test
+    async def test_enter(self):
+        class DefaultEnter(AbstractAsyncContextManager):
+            async def __aexit__(self, *args):
+                await super().__aexit__(*args)
+
+        manager = DefaultEnter()
+        self.assertIs(await manager.__aenter__(), manager)
+
+    def test_exit_is_abstract(self):
+        class MissingAexit(AbstractAsyncContextManager):
+            pass
+
+        with self.assertRaises(TypeError):
+            MissingAexit()
+
+    def test_structural_subclassing(self):
+        class ManagerFromScratch:
+            async def __aenter__(self):
+                return self
+            async def __aexit__(self, exc_type, exc_value, traceback):
+                return None
+
+        self.assertTrue(issubclass(ManagerFromScratch, AbstractAsyncContextManager))
+
+        class DefaultEnter(AbstractAsyncContextManager):
+            async def __aexit__(self, *args):
+                await super().__aexit__(*args)
+
+        self.assertTrue(issubclass(DefaultEnter, AbstractAsyncContextManager))
+
+
 class AsyncContextManagerTestCase(unittest.TestCase):
 
     @_async_test