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bpo-41543: contextlib.nullcontext can fill in for an async context manager #21870

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Merged
merged 2 commits into from
Nov 9, 2020
Merged

bpo-41543: contextlib.nullcontext can fill in for an async context manager #21870

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bpo-41543: contextlib.nullcontext can fill in for an async context ma…
tomgrin10 Aug 18, 2020
78d6808
Update Doc/library/contextlib.rst
asvetlov Nov 9, 2020
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18 changes: 18 additions & 0 deletions Doc/library/contextlib.rst
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Original file line number Diff line number Diff line change
Expand Up @@ -218,8 +218,26 @@ Functions and classes provided:
with cm as file:
# Perform processing on the file

It can also be used as a stand-in for
:ref:`asynchronous context managers <async-context-managers>`::

async def send_http(session=None):
if not session:
# If no http session, create it with aiohttp
cm = aiohttp.ClientSession()
else:
# Caller is responsible for closing the session
cm = nullcontext(session)

async with cm as session:
# Send http requests with session

.. versionadded:: 3.7

.. versionchanged:: 3.10
:term:`asynchronous context manager` support was added.



.. function:: suppress(*exceptions)

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8 changes: 7 additions & 1 deletion Lib/contextlib.py
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Expand Up @@ -681,7 +681,7 @@ def _fix_exception_context(new_exc, old_exc):
return received_exc and suppressed_exc


class nullcontext(AbstractContextManager):
class nullcontext(AbstractContextManager, AbstractAsyncContextManager):
"""Context manager that does no additional processing.

Used as a stand-in for a normal context manager, when a particular
Expand All @@ -700,3 +700,9 @@ def __enter__(self):

def __exit__(self, *excinfo):
pass

async def __aenter__(self):
return self.enter_result

async def __aexit__(self, *excinfo):
pass
14 changes: 13 additions & 1 deletion Lib/test/test_contextlib_async.py
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@@ -1,5 +1,7 @@
import asyncio
from contextlib import aclosing, asynccontextmanager, AbstractAsyncContextManager, AsyncExitStack
from contextlib import (
asynccontextmanager, AbstractAsyncContextManager,
AsyncExitStack, nullcontext, aclosing)
import functools
from test import support
import unittest
Expand Down Expand Up @@ -510,5 +512,15 @@ async def suppress_exc(*exc_details):
self.assertIsInstance(inner_exc.__context__, ZeroDivisionError)


class TestAsyncNullcontext(unittest.TestCase):
@_async_test
async def test_async_nullcontext(self):
class C:
pass
c = C()
async with nullcontext(c) as c_in:
self.assertIs(c_in, c)


if __name__ == '__main__':
unittest.main()
1 change: 1 addition & 0 deletions Misc/NEWS.d/next/Library/2020-08-14-00-39-04.bpo-41543.RpcRjb.rst
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@@ -0,0 +1 @@
Add async context manager support for contextlib.nullcontext.