typing_extensions.Protocol and typing.Protocol don't always play well together

[AlexWaygood]

If you're just using typing_extensions.Protocol, we backport python/cpython#31628 from Python 3.11. That means you can do this (the __init__ method on a protocol is retained, and can be used in concrete subclasses of that protocol):

>>> import typing_extensions as te
>>> class P(te.Protocol):
...     x: int
...     def __init__(self, x: int) -> None:
...         self.x = x
...
>>> class C(P): pass
...
>>> C(1).x
1

But, if you have both typing.Protocol and typing_extensions.Protocol in the mro, it doesn't work. On Python 3.8 (and, I assume, 3.9/3.10), there's this behaviour currently:

>>> import typing as t, typing_extensions as te
>>> class P(t.Protocol): pass
...
>>> class Q(P, te.Protocol):
...     x: int
...     def __init__(self, x: int) -> None:
...         self.x = x
...
>>> class C(Q): pass
...
>>> C(1).x
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'C' object has no attribute 'x'

I've been looking at it, and I think this may not be fixable, unfortunately. Should we document that this is a known limitation?

[JelleZijlstra]

Thanks for finding this.

It does look hard to fix. typing.Protocol before 3.11 clobbered __init__, so if you inherit from it, that's the behavior you'll get. We should simply document it, unless someone comes up with a clever way to prevent this behavior.

[AlexWaygood]

Another limitation: with typing_extensions.Protocol, it is possible to create a protocol that extends typing_extensions.Buffer:

>>> import typing_extensions as te
>>> class Q(te.Buffer, te.Protocol): pass
...
>>>

But if you have both typing.Protocol and typing_extensions.Protocol in the mro, it doesn't work:

>>> import typing as t, typing_extensions as te
>>> class P(t.Protocol): pass
...
>>> class Q(P, te.Buffer, te.Protocol): pass
...
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "C:\Users\alexw\coding\typing_extensions\src\typing_extensions.py", line 632, in __new__
    return abc.ABCMeta.__new__(mcls, name, bases, namespace, **kwargs)
  File "C:\Users\alexw\.conda\envs\py38\lib\abc.py", line 85, in __new__
    cls = super().__new__(mcls, name, bases, namespace, **kwargs)
  File "C:\Users\alexw\.conda\envs\py38\lib\typing.py", line 1119, in __init_subclass__
    raise TypeError('Protocols can only inherit from other'
TypeError: Protocols can only inherit from other protocols, got <class 'typing_extensions.Buffer'>

This is because we have typing_extensions.Buffer in our version of _PROTO_ALLOWLIST, but it obviously doesn't exist in the typing version of _PROTO_ALLOWLIST:

typing_extensions/src/typing_extensions.py

Lines 493 to 500 in fd1f853

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