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{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"Cannot use multiple inheritance with `collections.abc.Buffer` and `typing.Protocol`","articleBody":"Various stdlib classes are treated as protocols by type checkers, but are actually ABCs at runtime (for performance reasons). Examples include `contextlib.AbstractContextManager` and `collections.abc.Iterable`. These classes are special-cased in `typing.py` to allow for multiple inheritance with `typing.Protocol`, so that the interface can be extended:\r\n\r\n```pycon\r\n\u003e\u003e\u003e from contextlib import AbstractContextManager\r\n\u003e\u003e\u003e from typing import Protocol\r\n\u003e\u003e\u003e class Foo(AbstractContextManager, Protocol):\r\n...     def extra_method(self) -\u003e None: ...\r\n...\r\n\u003e\u003e\u003e\r\n```\r\n\r\n`collections.abc.Buffer` is a new-in-3.12 class that, like `AbstractContextManager` and `Iterable`, is an ABC at runtime but will be treated by type checkers as if it were a `Protocol`. However, multiple inheritance with `collections.abc.Buffer` and `typing.Protocol` currently fails:\r\n\r\n```pycon\r\n\u003e\u003e\u003e class Bar(Buffer, Protocol):\r\n...     def extra_method(self) -\u003e None: ...\r\n...\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\\abc.py\", line 106, in __new__\r\n    cls = super().__new__(mcls, name, bases, namespace, **kwargs)\r\n          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^\r\n  File \"C:\\Users\\alexw\\coding\\cpython\\Lib\\typing.py\", line 1916, in __init_subclass__\r\n    raise TypeError('Protocols can only inherit from other'\r\nTypeError: Protocols can only inherit from other protocols, got \u003cclass 'collections.abc.Buffer'\u003e\r\n```\r\n\r\nI think `Buffer` should be special-cased in the same way as `Buffer` and `Iterable`. It needs to be added to this mapping, I think:\r\n\r\nhttps://github.com/python/cpython/blob/ddb14859535ab8091381b9d0baf32dbe245b5e65/Lib/typing.py#L1740-L1746\r\n\r\nCc. @JelleZijlstra for PEP-688\n\n\u003c!-- gh-linked-prs --\u003e\n### Linked PRs\n* gh-104827\n* gh-104841\n\u003c!-- /gh-linked-prs --\u003e\n","author":{"url":"https://github.com/AlexWaygood","@type":"Person","name":"AlexWaygood"},"datePublished":"2023-05-23T14:18:02.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":0},"url":"https://github.com/104797/cpython/issues/104797"}