Various stdlib classes are treated as protocols by type checkers, but are actually ABCs at runtime (for performance reasons). Examples includecontextlib.AbstractContextManager andcollections.abc.Iterable. These classes are special-cased intyping.py to allow for multiple inheritance withtyping.Protocol, so that the interface can be extended:

>>>from contextlibimport AbstractContextManager>>>from typingimport Protocol>>>classFoo(AbstractContextManager,Protocol):...defextra_method(self) ->None:......>>>

collections.abc.Buffer is a new-in-3.12 class that, likeAbstractContextManager andIterable, is an ABC at runtime but will be treated by type checkers as if it were aProtocol. However, multiple inheritance withcollections.abc.Buffer andtyping.Protocol currently fails:

>>>classBar(Buffer,Protocol):...defextra_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 thinkBuffer should be special-cased in the same way asBuffer andIterable. It needs to be added to this mapping, I think:

Cc.@JelleZijlstra forPEP-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