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{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"multi-threading + fork warning when threads are stopped before fork","articleBody":"# Bug report\n\n### Bug description:\n\nIf all threads are stopped before a fork, and then restarted in the parent (and optionally the child) process, the new multi-threading with fork warning is given\n\n```python\nimport os\nimport threading\n\n\nstop = False\n\n\ndef target():\n    while not stop:\n        pass\n\n\nthread = threading.Thread(target=target)\n\n\ndef stop_thread():\n    global stop\n    stop = True\n    thread.join()\n\n\ndef restart_thread():\n    global thread, stop\n    stop = False\n    (thread := threading.Thread(target=target)).start()\n\n\nos.register_at_fork(before=stop_thread, after_in_child=restart_thread, after_in_parent=restart_thread)\n\n\nthread.start()\n\nif os.fork() == 0:\n    os._exit(0)\n```\nIf the thread is not restarted in the parent, no warning is given. I believe `warn_about_fork_with_threads` should be refactored so that a warning is given to the user only if there are threads detected after the call to `PyOS_BeforeFork`. If it is not possible to give a warning there, as stated in the comments, perhaps one can just do the thread count check there and take note of the result, then warn after `PyOS_AfterFork_Parent`,  when safe to do so.\n\n### CPython versions tested on:\n\n3.13\n\n### Operating systems tested on:\n\nmacOS\n\n\u003c!-- gh-linked-prs --\u003e\n### Linked PRs\n* gh-141438\n* gh-141614\n* gh-141639\n\u003c!-- /gh-linked-prs --\u003e\n","author":{"url":"https://github.com/P403n1x87","@type":"Person","name":"P403n1x87"},"datePublished":"2025-07-25T16:35:11.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":9},"url":"https://github.com/137109/cpython/issues/137109"}