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{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"Typing: undocumented behaviour change for protocols decorated with `@final` and `@runtime_checkable` in 3.11","articleBody":"Python 3.11 introduced an undocumented behaviour change for protocols decorated with both `@final` and `@runtime_checkable`. On 3.10:\r\n\r\n```pycon\r\n\u003e\u003e\u003e from typing import *\r\n\u003e\u003e\u003e @final\r\n... @runtime_checkable\r\n... class Foo(Protocol):\r\n...     def bar(self): ...\r\n...\r\n\u003e\u003e\u003e class Spam:\r\n...     def bar(self): ...\r\n...\r\n\u003e\u003e\u003e issubclass(Spam, Foo)\r\nTrue\r\n\u003e\u003e\u003e isinstance(Spam(), Foo)\r\nTrue\r\n```\r\n\r\nOn 3.11:\r\n\r\n```pycon\r\n\u003e\u003e\u003e from typing import *\r\n\u003e\u003e\u003e @final\r\n... @runtime_checkable\r\n... class Foo(Protocol):\r\n...     def bar(self): ...\r\n...\r\n\u003e\u003e\u003e class Spam:\r\n...     def bar(self): ...\r\n...\r\n\u003e\u003e\u003e issubclass(Spam, Foo)\r\nTraceback (most recent call last):\r\n  File \"\u003cstdin\u003e\", line 1, in \u003cmodule\u003e\r\n  File \"\u003cfrozen abc\u003e\", line 123, in __subclasscheck__\r\n  File \"C:\\Users\\alexw\\coding\\cpython\\Lib\\typing.py\", line 1547, in _proto_hook\r\n    raise TypeError(\"Protocols with non-method members\"\r\n    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^\r\nTypeError: Protocols with non-method members don't support issubclass()\r\n\u003e\u003e\u003e isinstance(Spam(), Foo)\r\nFalse\r\n```\r\n\r\nThis is because, following 0bbf30e (by @JelleZijlstra), the `@final` decorator sets a `__final__` attribute wherever it can, so that it is introspectable by runtime tools. But the runtime-checkable-protocol `isinstance()` machinery doesn't know anything about the `__final__` attribute, so it assumes that the `__final__` attribute is just a regular protocol member.\r\n\r\nThis should be pretty easy to fix: we just need to add `__final__` to the set of \"special attributes\" ignored by runtime-checkable-protocol `isinstance()`/`issubclass()` checks here:\r\n\r\nhttps://github.com/python/cpython/blob/848bdbe166b71ab2ac2c0c1d88432fb995d1444c/Lib/typing.py#L1906-L1909\r\n\r\n@JelleZijlstra do you agree with that course of action?\n\n\u003c!-- gh-linked-prs --\u003e\n### Linked PRs\n* gh-103173\n* gh-105445\n* gh-105473\n* gh-105474\n\u003c!-- /gh-linked-prs --\u003e\n","author":{"url":"https://github.com/AlexWaygood","@type":"Person","name":"AlexWaygood"},"datePublished":"2023-04-01T11:46:48.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":14},"url":"https://github.com/103171/cpython/issues/103171"}