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{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"_socket extension C capsule cannot be visited by the GC and so _socket.socket type may stay alive longer than expected","articleBody":"# Bug report\n\n### Checklist\n\n- [X] I am confident this is a bug in CPython, not a bug in a third-party project\n- [X] I have searched the [CPython issue tracker](https://github.com/python/cpython/issues?q=is%3Aissue+sort%3Acreated-desc),\nand am confident this bug has not been reported before\n\n\n### CPython versions tested on:\n\nCPython main branch\n\n### Operating systems tested on:\n\nLinux\n\n### Output from running 'python -VV' on the command line:\n\nPython 3.13.0a0 (heads/main:d63972e289, Aug 21 2023, 22:29:24) [GCC 13.2.1 20230728 (Red Hat 13.2.1-1)]\n\n### A clear and concise description of the bug:\n\nThe following script will never delete ``_socket.socket`` type:\n\n```python\nimport _socket\n\n# uncomment to workaround the bug\n#_socket.CAPI = None\n\n_socket = None\n\nimport sys\ndel sys.modules['_socket']\n\nimport gc\ngc.collect()\n\nprint(\"exit\")\n```\n\nExample on a Python debug build:\n\n```sh\n$ ./python -X showrefcount script.py \nexit\n[408 refs, 260 blocks]\n```\n\nThe GC cannot visit the strong reference to the type stored in ``_socket.CAPI`` capsule (C API). In Python, creating a heap type creates a reference cycle. For example, a type MRO contains the type. Methods also contain a strong reference to their type.\n\n```py\n$ ./python\nPython 3.13.0a0 (heads/main:d63972e289, Aug 21 2023, 22:29:24) [GCC 13.2.1 20230728 (Red Hat 13.2.1-1)] on linux\n\u003e\u003e\u003e import _socket\n\u003e\u003e\u003e t=_socket.socket\n\u003e\u003e\u003e t.__mro__[0] is t\nTrue\n```\n\nSee also:\n\n* https://pythondev.readthedocs.io/garbage_collector.html#reference-cycles\n* https://vstinner.github.io/debug-python-refleak.html\n* https://vstinner.github.io/subinterpreter-leaks.html\n\n\u003c!-- gh-linked-prs --\u003e\n### Linked PRs\n* gh-108241\n* gh-108339\n* gh-108417\n* gh-108596\n\u003c!-- /gh-linked-prs --\u003e\n","author":{"url":"https://github.com/vstinner","@type":"Person","name":"vstinner"},"datePublished":"2023-08-21T20:46:24.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":19},"url":"https://github.com/108240/cpython/issues/108240"}