bpo-30266: support "= None" pattern in AbstractContextManager

Lib/contextlib.py

@@ -1,6 +1,7 @@
 """Utilities for with-statement contexts. See PEP 343."""
 import abc
 import sys
+import _collections_abc
 from collections import deque
 from functools import wraps
 
@@ -25,9 +26,7 @@ def __exit__(self, exc_type, exc_value, traceback):
  @classmethod
  def __subclasshook__(cls, C):
  if cls is AbstractContextManager:
- 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 _collections_abc._check_methods(C, "__enter__", "__exit__")
  return NotImplemented
 
 

Lib/test/test_contextlib.py

@@ -44,6 +44,16 @@ def __exit__(self, *args):
 
  self.assertTrue(issubclass(DefaultEnter, AbstractContextManager))
 
+ class NoEnter(ManagerFromScratch):
+ __enter__ = None
+
+ self.assertFalse(issubclass(NoEnter, AbstractContextManager))
+
+ class NoExit(ManagerFromScratch):
+ __exit__ = None
+
+ self.assertFalse(issubclass(NoExit, AbstractContextManager))
+
 
 class ContextManagerTestCase(unittest.TestCase):
 

Misc/NEWS

@@ -363,6 +363,10 @@ Library
 - bpo-30048: Fixed ``Task.cancel()`` can be ignored when the task is
  running coroutine and the coroutine returned without any more ``await``.
 
+- bpo-30266: contextlib.AbstractContextManager now supports anti-registration
+ by setting __enter__ = None or __exit__ = None, following the pattern
+ introduced in bpo-25958. Patch by Jelle Zijlstra.
+
 - bpo-30340: Enhanced regular expressions optimization. This increased
  the performance of matching some patterns up to 25 times.