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:

>>> from contextlib import AbstractContextManager >>> from typing import Protocol >>> class Foo(AbstractContextManager, Protocol): ... def extra_method(self) -> None: ... ... >>>

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:

>>> class Bar(Buffer, Protocol): ... def extra_method(self) -> None: ... ... Traceback (most recent call last): File "<stdin>", line 1, in <module> File "C:\Users\alexw\coding\cpython\Lib\abc.py", line 106, in __new__ cls = super().__new__(mcls, name, bases, namespace, **kwargs) ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ File "C:\Users\alexw\coding\cpython\Lib\typing.py", line 1916, in __init_subclass__ raise TypeError('Protocols can only inherit from other' TypeError: Protocols can only inherit from other protocols, got <class 'collections.abc.Buffer'>

I think Buffer should be special-cased in the same way as Buffer and Iterable. It needs to be added to this mapping, I think:

Cc. @JelleZijlstra for PEP-688

Linked PRs

• gh-104797: Allow Protocols to inherit from collections.abc.Buffer #104827
• [3.12] gh-104797: Allow Protocols to inherit from collections.abc.Buffer (GH-104827) #104841