__match_args__ + runtime_checkable protocol

[sobolevn]

Bug report

Since __match_args__ is not mentioned in _SPECIAL_NAMES right now these two examples have different results:

@runtime_checkable
class P(Protocol):
    x: int
    y: int

class A:
    def __init__(self, x: int, y: int):
        self.x = x
        self.y = y

assert isinstance(A(1, 2), P) is True

And:

@runtime_checkable
class P(Protocol):
    __match_args__ = ('x', 'y')
    x: int
    y: int

class A:
    def __init__(self, x: int, y: int):
        self.x = x
        self.y = y

assert isinstance(A(1, 2), P) is False

Why I think that __match_args__ is a special attribute and should not be checked in isinstance?

1. It might be useed for protocol itself in patma (new issue is on its way about it):

match A(1, 2):
    case P(x, y):
        print(x, y)

But, this does not work right now if A does not have __match_args__. Which is not really required for this case.

2. Several similar attributes like __slots__ and __weakref__ and __annotations__ are ignored already

Do others agree?
CC @AlexWaygood for @runtime_checkable protocols

I will send a PR with my proposed solution and tests :)

Linked PRs

• gh-110682: Ignore __match_args__ from __instancecheck__ in protocols #110683

[sobolevn]

Related #110686

[gvanrossum]

I agree that __match_args__ ought to be disregarded by isinstance() in this case.

[AlexWaygood]

Sounds like the consensus is that this was really a "missing feature" rather than a bug, so we won't be backporting this