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{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"`ThreadingMock.call_count` is not thread safe","articleBody":"# Bug report\n\n### Bug description:\n\nRunning this test program with (plain old, non-free-threaded) Python 3.14 fails:\n```python\nimport threading\nimport unittest.mock\n\nm = unittest.mock.ThreadingMock()\n\nLOOPS = 10_000\nTHREADS = 10\n\n\ndef test_function():\n    for _ in range(LOOPS):\n        m()\n\n\nthreads = [threading.Thread(target=test_function) for _ in range(THREADS)]\nfor thread in threads:\n    thread.start()\nfor thread in threads:\n    thread.join()\n\nassert m.call_count == LOOPS * THREADS, f\"Expected {LOOPS * THREADS}, got {m.call_count}\"\n```\n\nResults are something like:\n```sh\n$ python3.14 python_mock_call_count_atomicity.py\nTraceback (most recent call last):\n  File \"python_mock_call_count_atomicity.py\", line 21, in \u003cmodule\u003e\n    assert m.call_count == LOOPS * THREADS, f\"Expected {LOOPS * THREADS}, got {m.call_count}\"\n           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^\nAssertionError: Expected 100000, got 95507\n```\n\nThis 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.\n\n### CPython versions tested on:\n\n3.14\n\n### Operating systems tested on:\n\nLinux\n\n\u003c!-- gh-linked-prs --\u003e\n### Linked PRs\n* gh-142656\n* gh-142674\n* gh-142743\n* gh-142744\n* gh-142922\n* gh-145739\n* gh-145740\n\u003c!-- /gh-linked-prs --\u003e\n","author":{"url":"https://github.com/godlygeek","@type":"Person","name":"godlygeek"},"datePublished":"2025-12-12T21:46:52.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":9},"url":"https://github.com/142651/cpython/issues/142651"}