[spec] Formalize excluded Protocol members

[randolf-scholz]

At runtime, the following members are currently excluded by Protocol: (source)

_TYPING_INTERNALS = frozenset({
    '__parameters__', '__orig_bases__',  '__orig_class__',
    '_is_protocol', '_is_runtime_protocol', '__protocol_attrs__',
    '__non_callable_proto_members__', '__type_params__',
})


_SPECIAL_NAMES = frozenset({
    '__abstractmethods__', '__annotations__', '__dict__', '__doc__',
    '__init__', '__module__', '__new__', '__slots__',
    '__subclasshook__', '__weakref__', '__class_getitem__',
    '__match_args__', '__static_attributes__', '__firstlineno__',
    '__annotate__',
})


# These special attributes will be not collected as protocol members.
EXCLUDED_ATTRIBUTES = _TYPING_INTERNALS | _SPECIAL_NAMES | {'_MutableMapping__marker'}

However, neither PEP 544 nor the typing spec formalize this list, which leads to diverging behavior between different type checkers:

Code sample in pyright playground, mypy playground

from typing import TypeIs, Protocol

class MyProtocol(Protocol):
    @classmethod
    def __subclasshook__(cls, other: type, /) -> TypeIs[type["MyProtocol"]]:
        ...

x: MyProtocol = int(1)  # mypy: ✅ pyright: ❌

Code sample in pyright playground, mypy playground

from typing import Protocol

class MyProtocol(Protocol):
    __abstractmethods__: frozenset[str]
    
x: MyProtocol = int(1)  # mypy: ✅ pyright: ❌

A real-world example where this matters is for instance a Protocol for NamedTuple Instances, that checks whether types.get_original_bases(cls) includes typing.NamedTuple inside __subclasshook__.

[erictraut]

I think this would be a good addition to the spec. @randolf-scholz, would you be willing to write a proposed modification to the spec and start a discussion thread to solicit comments from the broader community?

[randolf-scholz]

I can post something in discourse in the next days.