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{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"Non-runtime_checkable subclass of a runtime_checkable Protocol allows isinstance()","articleBody":"# Bug report\n\n### Bug description:\n\nI would expect this to raise, but it passes on all versions:\n\n```py\nfrom typing import runtime_checkable, Protocol\n@runtime_checkable\nclass Foo(Protocol):\n    x: int\nclass Bar(Foo, Protocol):\n    y: str\nisinstance(object(), Bar)\n```\n\nSome previous discussion in https://github.com/python/cpython/pull/132596#discussion_r2047194339 . We may not want to change this behavior for compatibility reasons, but I think it's a bug so I'm at least opening the issue.\n\n### CPython versions tested on:\n\nCPython main branch, 3.13\n\n### Operating systems tested on:\n\nmacOS\n\n\u003c!-- gh-linked-prs --\u003e\n### Linked PRs\n* gh-143806\n\u003c!-- /gh-linked-prs --\u003e\n","author":{"url":"https://github.com/JelleZijlstra","@type":"Person","name":"JelleZijlstra"},"datePublished":"2025-04-16T16:43:11.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":1},"url":"https://github.com/132604/cpython/issues/132604"}