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mock.mock_open __exit__ called as instance method from contextlib.ExitStack context #150484
Description
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")):
passraises
TypeError: mock_open.<locals>._exit_side_effect() takes 3 positional arguments but 4 were given
The issue seems to come from 3f7c081
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:
- changing the way ExitStack calls its stack of __exit__ callbacks
- making mock_open and open more similar, or
- 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-150484: Fix mock_open __exit__ with contextlib.ExitStack #150521
- fix unittest.mock: make mock_open.__exit__ accept variable args (gh-150484) #150535
- [3.14] gh-150484: Fix mock_open __exit__ with contextlib.ExitStack #151829
- [3.13] gh-150484: Fix mock_open __exit__ with contextlib.ExitStack (GH-151829) #151861
Activity
Add news entry for pythongh-150484
add news fragment for pythongh-150484 mock_open fix
serhiy-storchaka commented on May 29, 2026
This was fixed in 3.15 (most likely in #144386), so only 3.13 and 3.14 need a fix (too late for 3.12, security bugs only).
Yes, using *args is a correct fix for these versions.
clarker0 commented on Jun 1, 2026
3.12 is working fine! sorry for the confusion, I added it to the "tested on" versions list in order to bisect where the issue came from.
only 3.13 and 3.14 are affected by this issue.
a fix for 3.14 would be hugely appreciated, thanks 😄
[3.14] gh-150484: Fix mock_open __exit__ with contextlib.ExitStack (G…
[3.13] gh-150484: Fix mock_open __exit__ with contextlib.ExitStack (G…