mock.mock_open __exit__ called as instance method from contextlib.ExitStack context

Bug report

Bug description:

In Python 3.13 onwards:

from contextlib import ExitStack
from unittest import mock
with mock.patch("__main__.open", mock.mock_open()) as m:
    with ExitStack() as exit_stack:
        with exit_stack.enter_context(open("/tmp/test.txt", "w")):
            pass

raises

TypeError: mock_open.<locals>._exit_side_effect() takes 3 positional arguments but 4 were given

The issue seems to come from https://github.com/python/cpython/commit/3f7c0810f6158a7ff37be432f8d7f9511427489f

It appears that in Python versions 3.12 and earlier, mock_open().__exit__ was always called with 4 args (self, exctype, excinst, exctb) when used with contextlib.ExitStack but that used to be a no-op before the exit handler was added.

I'm not sure whether this should be fixed by:

1. changing the way ExitStack calls its stack of __exit__ callbacks
2. making mock_open and open more similar, or
3. making mock_open's exit handler take *args, **kwargs same as the enter handler??

Thanks

CPython versions tested on:

3.12, 3.13, 3.14

Operating systems tested on:

Linux

Linked PRs

• gh-150521
• gh-150535
• gh-151829
• gh-151861

Begin with the reproducer using contextlib.ExitStack and unittest.mock.mock_open, then inspect the callback behavior described in the issue. Done means resolving the Python 3.13+ TypeError while preserving the expected context-manager behavior and adding coverage for the reported case.