`ThreadingMock.call_count` is not thread safe

[godlygeek]

Bug report

Bug description:

Running this test program with (plain old, non-free-threaded) Python 3.14 fails:

import threading
import unittest.mock

m = unittest.mock.ThreadingMock()

LOOPS = 10_000
THREADS = 10


def test_function():
    for _ in range(LOOPS):
        m()


threads = [threading.Thread(target=test_function) for _ in range(THREADS)]
for thread in threads:
    thread.start()
for thread in threads:
    thread.join()

assert m.call_count == LOOPS * THREADS, f"Expected {LOOPS * THREADS}, got {m.call_count}"

Results are something like:

$ python3.14 python_mock_call_count_atomicity.py
Traceback (most recent call last):
  File "python_mock_call_count_atomicity.py", line 21, in <module>
    assert m.call_count == LOOPS * THREADS, f"Expected {LOOPS * THREADS}, got {m.call_count}"
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
AssertionError: Expected 100000, got 95507

This is essentially the same bug as #122957, but that was closed with just a fix to the test suite. This is a bug report that the version of MagicMock for multithreading tests is not thread-safe and so the obvious way of writing those multithreading tests may not work.

CPython versions tested on:

3.14

Operating systems tested on:

Linux

Linked PRs

• gh-142651: Make call_count of Mock thread safe #142656
• gh-142651 demonstrate that the test fails on main #142674
• [3.14] gh-142651: make Mock.call_count thread-safe (GH-142656) #142743
• [3.13] gh-142651: make Mock.call_count thread-safe (GH-142656) #142744

[chaope]

https://docs.python.org/3/library/unittest.mock.html says:

call_args_list
This is a list of all the calls made to the mock object in sequence (so the length of the list is the number of times it has been called).

Probably we can just use the length of this list as call_count.

I'll open a PR with this solution.

[cjw296]

Does Mock document itself to be thread-safe? I'm not averse to bug fixes like this, but unless it's a clearly documented expectation that Mock is thread safe, I fear there may be many more cases like this lurking...

[kumaraditya303]

Does Mock document itself to be thread-safe? I'm not averse to bug fixes like this, but unless it's a clearly documented expectation that Mock is thread safe, I fear there may be many more cases like this lurking...

I created #142752 for followup on this