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{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"Strange interaction between `typing.Protocol` and unrelated `isinstance()` checks","articleBody":"On `main`, an `isinstance()` check against `Sized` works just the same as it does on Python 3.11:\r\n\r\n```pycon\r\n\u003e\u003e\u003e from typing import Sized\r\n\u003e\u003e\u003e isinstance(1, Sized)\r\nFalse\r\n```\r\n\r\nHowever! If you first subclass `Sized` like this, `TypeError` is raised on that `isinstance()` check!\r\n\r\n```pycon\r\n\u003e\u003e\u003e from typing import Sized, Protocol\r\n\u003e\u003e\u003e class Foo(Sized, Protocol): pass\r\n...\r\n\u003e\u003e\u003e isinstance(1, Sized)\r\nTraceback (most recent call last):\r\n  File \"\u003cstdin\u003e\", line 1, in \u003cmodule\u003e\r\n  File \"C:\\Users\\alexw\\coding\\cpython\\Lib\\typing.py\", line 1153, in __instancecheck__\r\n    return self.__subclasscheck__(type(obj))\r\n           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^\r\n  File \"C:\\Users\\alexw\\coding\\cpython\\Lib\\typing.py\", line 1428, in __subclasscheck__\r\n    return issubclass(cls, self.__origin__)\r\n           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^\r\n  File \"\u003cfrozen abc\u003e\", line 123, in __subclasscheck__\r\n  File \"C:\\Users\\alexw\\coding\\cpython\\Lib\\typing.py\", line 1793, in __subclasscheck__\r\n    return super().__subclasscheck__(other)\r\n           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^\r\n  File \"\u003cfrozen abc\u003e\", line 123, in __subclasscheck__\r\n  File \"C:\\Users\\alexw\\coding\\cpython\\Lib\\typing.py\", line 1876, in _proto_hook\r\n    raise TypeError(\"Instance and class checks can only be used with\"\r\nTypeError: Instance and class checks can only be used with @runtime_checkable protocols\r\n```\r\n\r\nThis was originally reported by @vnmabus in https://github.com/python/typing_extensions/issues/207.\r\n\r\nNote that (because of the `abc`-module cache), this _doesn't_ reproduce if you do an `isinstance()` check _before_ subclassing `typing.Sized`:\r\n\r\n```pycon\r\n\u003e\u003e\u003e import typing\r\n\u003e\u003e\u003e isinstance(1, typing.Sized)\r\nFalse\r\n\u003e\u003e\u003e class Foo(typing.Sized, typing.Protocol): pass\r\n...\r\n\u003e\u003e\u003e isinstance(1, typing.Sized)\r\nFalse\r\n```\n\n\u003c!-- gh-linked-prs --\u003e\n### Linked PRs\n* gh-105152\n* gh-105159\n* gh-105160\n\u003c!-- /gh-linked-prs --\u003e\n","author":{"url":"https://github.com/AlexWaygood","@type":"Person","name":"AlexWaygood"},"datePublished":"2023-05-31T14:30:55.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":4},"url":"https://github.com/105144/cpython/issues/105144"}