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{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"Some pre-finalization callbacks can create other callbacks","articleBody":"# Bug report\n\n### Bug description:\n\nCurrently, \"pre-finalization\" (callbacks executed while the interpreter is still fully intact) looks like this:\n\nhttps://github.com/python/cpython/blob/642e5dfc74310d15bb81f8e94167590380a5fbfb/Python/pylifecycle.c#L2023-L2038\n\nThreads are joined first, then pending calls are executed, and finally atexit callbacks are executed. The problem is that any of these three can create one another, such as a pending call creating a thread, or an atexit handler scheduling a pending call. The best way to demonstrate this is by creating a new thread inside of an atexit callback:\n\n```python\nimport atexit\nimport threading\nimport time\n\n\ndef run():\n    print(24)\n    time.sleep(1)\n    print(42)\n\n@atexit.register\ndef start_thread():\n    threading.Thread(target=run).start()\n```\n\nOn 3.13+ (and probably earlier), you'll get one of these three outputs:\n\n1. `24` is printed.\n2. Nothing is printed.\n3. A fatal error occurs due to the stdout lock being unavailable.\n\nThis is because the thread created is incorrectly non-daemon, because the call to `threading._shutdown` (which should join it) has already happened.\n\n@ericsnowcurrently, I discussed this with you at the PyCon sprints. I think we came to the agreement that the best way to fix this is by looping until all three of them execute no callbacks.\n\n### CPython versions tested on:\n\nCPython main branch\n\n### Operating systems tested on:\n\nLinux\n\n\u003c!-- gh-linked-prs --\u003e\n### Linked PRs\n* gh-136004\n* gh-139129\n* gh-139225\n\u003c!-- /gh-linked-prs --\u003e\n","author":{"url":"https://github.com/ZeroIntensity","@type":"Person","name":"ZeroIntensity"},"datePublished":"2025-06-26T22:43:00.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":8},"url":"https://github.com/136003/cpython/issues/136003"}