diff --git a/Doc/library/contextlib.rst b/Doc/library/contextlib.rst
index 0aa4ad76523480..6872de382e897c 100644
--- a/Doc/library/contextlib.rst
+++ b/Doc/library/contextlib.rst
@@ -126,6 +126,31 @@ Functions and classes provided:
.. versionadded:: 3.7
+ Context managers defined with :func:`asynccontextmanager` can be used
+ either as decorators or with :keyword:`async with` statements::
+
+ import time
+
+ async def timeit():
+ now = time.monotonic()
+ try:
+ yield
+ finally:
+ print(f'it took {time.monotonic() - now}s to run')
+
+ @timeit()
+ async def main():
+ # ... async code ...
+
+ When used as a decorator, a new generator instance is implicitly created on
+ each function call. This allows the otherwise "one-shot" context managers
+ created by :func:`asynccontextmanager` to meet the requirement that context
+ managers support multiple invocations in order to be used as decorators.
+
+ .. versionchanged:: 3.10
+ Async context managers created with :func:`asynccontextmanager` can
+ be used as decorators.
+
.. function:: closing(thing)
@@ -351,6 +376,43 @@ Functions and classes provided:
.. versionadded:: 3.2
+.. class:: AsyncContextManager
+
+ Similar as ContextManger only for async
+
+ Example of ``ContextDecorator``::
+
+ from asyncio import run
+ from contextlib import AsyncContextDecorator
+
+ class mycontext(AsyncContextDecorator):
+ async def __aenter__(self):
+ print('Starting')
+ return self
+
+ async def __aexit__(self, *exc):
+ print('Finishing')
+ return False
+
+ >>> @mycontext()
+ ... async def function():
+ ... print('The bit in the middle')
+ ...
+ >>> run(function())
+ Starting
+ The bit in the middle
+ Finishing
+
+ >>> async def function():
+ ... async with mycontext():
+ ... print('The bit in the middle')
+ ...
+ >>> run(function())
+ Starting
+ The bit in the middle
+ Finishing
+
+
.. class:: ExitStack()
A context manager that is designed to make it easy to programmatically
diff --git a/Lib/contextlib.py b/Lib/contextlib.py
index ff92d9f913f4c2..6838bea96a96c6 100644
--- a/Lib/contextlib.py
+++ b/Lib/contextlib.py
@@ -80,6 +80,22 @@ def inner(*args, **kwds):
return inner
+class AsyncContextDecorator(object):
+ "A base class or mixin that enables async context managers to work as decorators."
+
+ def _recreate_cm(self):
+ """Return a recreated instance of self.
+ """
+ return self
+
+ def __call__(self, func):
+ @wraps(func)
+ async def inner(*args, **kwds):
+ async with self._recreate_cm():
+ return await func(*args, **kwds)
+ return inner
+
+
class _GeneratorContextManagerBase:
"""Shared functionality for @contextmanager and @asynccontextmanager."""
@@ -167,9 +183,16 @@ def __exit__(self, type, value, traceback):
class _AsyncGeneratorContextManager(_GeneratorContextManagerBase,
- AbstractAsyncContextManager):
+ AbstractAsyncContextManager,
+ AsyncContextDecorator):
"""Helper for @asynccontextmanager."""
+ def _recreate_cm(self):
+ # _AGCM instances are one-shot context managers, so the
+ # ACM must be recreated each time a decorated function is
+ # called
+ return self.__class__(self.func, self.args, self.kwds)
+
async def __aenter__(self):
try:
return await self.gen.__anext__()
diff --git a/Lib/test/test_contextlib_async.py b/Lib/test/test_contextlib_async.py
index 43fb7fced1bfdb..a631218e158389 100644
--- a/Lib/test/test_contextlib_async.py
+++ b/Lib/test/test_contextlib_async.py
@@ -278,6 +278,33 @@ async def woohoo(self, func, args, kwds):
async with woohoo(self=11, func=22, args=33, kwds=44) as target:
self.assertEqual(target, (11, 22, 33, 44))
+ @_async_test
+ async def test_recursive(self):
+ depth = 0
+ ncols = 0
+
+ @asynccontextmanager
+ async def woohoo():
+ nonlocal ncols
+ ncols += 1
+
+ nonlocal depth
+ before = depth
+ depth += 1
+ yield
+ depth -= 1
+ self.assertEqual(depth, before)
+
+ @woohoo()
+ async def recursive():
+ if depth < 10:
+ await recursive()
+
+ await recursive()
+
+ self.assertEqual(ncols, 10)
+ self.assertEqual(depth, 0)
+
class TestAsyncExitStack(TestBaseExitStack, unittest.TestCase):
class SyncAsyncExitStack(AsyncExitStack):
diff --git a/Misc/NEWS.d/next/Library/2020-05-29-15-25-41.bpo-40816.w61Pob.rst b/Misc/NEWS.d/next/Library/2020-05-29-15-25-41.bpo-40816.w61Pob.rst
new file mode 100644
index 00000000000000..66b75779784655
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2020-05-29-15-25-41.bpo-40816.w61Pob.rst
@@ -0,0 +1 @@
+Add AsyncContextDecorator to contextlib to support async context manager as a decorator.
\ No newline at end of file
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