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

Co-authored-by: Andrew Svetlov <andrew.svetlov@gmail.com>

2 files changed, 20 insertions, 2 deletions

diff --git a/Lib/contextlib.py b/Lib/contextlib.py
index 56b4968..a0b523c 100644
--- a/Lib/contextlib.py
+++ b/Lib/contextlib.py
@@ -704,7 +704,7 @@ class AsyncExitStack(_BaseExitStack, AbstractAsyncContextManager):
         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
@@ -723,3 +723,9 @@ class nullcontext(AbstractContextManager):
 
     def __exit__(self, *excinfo):
         pass
+
+    async def __aenter__(self):
+        return self.enter_result
+
+    async def __aexit__(self, *excinfo):
+        pass
diff --git a/Lib/test/test_contextlib_async.py b/Lib/test/test_contextlib_async.py
index 109807d..290ef05 100644
--- a/Lib/test/test_contextlib_async.py
+++ b/Lib/test/test_contextlib_async.py
@@ -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
@@ -537,5 +539,15 @@ class TestAsyncExitStack(TestBaseExitStack, unittest.TestCase):
         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()