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{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"TypeAliasType can leak memory if name is a str subclass","articleBody":"# Bug report\n\n### Bug description:\n\nTypeAliasType's traverse implementation does not visit its name. This means that we can leak memory if name is passed an instance of a mutable str subclass.\n\n```python\nfrom typing import TypeAliasType\nimport weakref\nimport gc\n\nclass S(str):\n    pass\n\ndef still_alive(ref):\n    obj = ref()\n    if not isinstance(obj, S):\n        return False\n    T = obj.T\n    if not isinstance(T, TypeAliasType):\n        return False\n    return T\n\nname = S(\"name\")\nT = TypeAliasType(name, int)\nname.T = T\nref = weakref.ref(name)\nassert still_alive(ref)\ndel name\ndel T\ngc.collect()\ngc.collect()\nassert still_alive(ref)\n\ndel ref().T\nassert not still_alive(ref)\n\n```\n\n\n\n### CPython versions tested on:\n\nCPython main branch\n\n### Operating systems tested on:\n\nmacOS\n\n\u003c!-- gh-linked-prs --\u003e\n### Linked PRs\n* gh-140016\n* gh-140063\n* gh-140073\n\u003c!-- /gh-linked-prs --\u003e\n","author":{"url":"https://github.com/JelleZijlstra","@type":"Person","name":"JelleZijlstra"},"datePublished":"2025-10-12T14:45:22.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":2},"url":"https://github.com/140000/cpython/issues/140000"}