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How should structural subtyping work for objects with slots but no variable declarations? #1367
Description
Consider the snippet below:
from typing import runtime_checkable, Protocol
@runtime_checkable
class HasX(Protocol):
x: int
class HasNothingButSlots:
__slots__ = ("x",)
def requires_object_with_x(obj: HasX) -> None:
print(obj.x)
requires_object_with_x(HasNothingButSlots())Mypy does not see HasNothingButSlots as a valid subtype of of HasX, so complains about this code. Pyright, however, does see HasNothingButSlots as a valid subtype of HasX, so has no complaints.
Currently, the runtime agrees with mypy: calling isinstance(HasNothingButSlots(), HasX) evaluates to False (due to the fact that the x variable has not been initialised on the instance). If we implement any of the patches discussed in #1363, this will change, and the runtime will agree with pyright: isinstance(HasNothingButSlots(), HasX) will evaluate to True even if the x variable has not been initialised on the instance.
Who is correct: mypy or pyright?
Activity
[-]How should structural subtyping work for objects with slots?[/-][+]How should structural subtyping work for objects with slots but no variable declarations?[/+]TeamSpen210 commentedon Mar 9, 2023
Perhaps it would be reasonable for mypy to treat this as
x: Any, but also produce a "x needs a type definition" or "Any type inferred" warning?gvanrossum commentedon Mar 10, 2023
I tend to agree with pyright -- surely any realistic class with slots does initialize the slots in
__init__or some other method?AlexWaygood commentedon Mar 12, 2023
The points made here makes sense to me. Seems like the consensus is that pyright's behaviour is preferable — closing as resolved.