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 for contextlib.ContextManager (as for other contextlib ABCs):

from typing import ContextManager

class A(ContextManager[int]):
    __slots__ = ()
    __enter__ = __exit__ = lambda *args: None
    
A().a = 1  # No exception

Changing ContextManager to MutableMapping, 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__ to AbstractContextManager and AbstractAsyncContextManager #106771