Cannot use multiple inheritance with collections.abc.Buffer and typing.Protocol

[AlexWaygood]

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:

cpython/Lib/typing.py

Lines 1740 to 1746 in ddb1485

	_PROTO_ALLOWLIST = {
	'collections.abc': [
	'Callable', 'Awaitable', 'Iterable', 'Iterator', 'AsyncIterable',
	'Hashable', 'Sized', 'Container', 'Collection', 'Reversible',
	],
	'contextlib': ['AbstractContextManager', 'AbstractAsyncContextManager'],
	}

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