Python's ABCs should have__slots__ attributes to not interfere with slotting in derived classes (especially when the class has no members). However, it is not the case forcontextlib.ContextManager (as for othercontextlib ABCs):

fromtypingimportContextManagerclassA(ContextManager[int]):__slots__= ()__enter__=__exit__=lambda*args:NoneA().a=1# No exception

ChangingContextManager toMutableMapping, for example (and setting relevant dunders), does raise an exception.

Observed on Ubuntu 20.04 and Python 3.9.16.

Linked PRs

• gh-105726: Add__slots__ toAbstractContextManager andAbstractAsyncContextManager #106771