Question: type annotating a `validator` callback

[jacopoabramo]

I'm trying to correctly type hint the validator callback function of a class declared via @define and I'm a bit confused. Given the following example simplified from the documentation:

from attrs import define

@define
class C:
    x: int
    
    @x.validator
    def fits_byte(self, attribute, value):
        if not 0 <= value < 256:
            raise ValueError("value out of bounds")

What's the correct annotation for fits_byte? At a glance, it looked to me like it should be:

from typing import TYPE_CHECKING
from attrs import define

if TYPE_CHECKING:
    from attrs import Attribute

@define
class C:
    x: int
    
    @x.validator
    def fits_byte(self, attribute: Attribute[int], value: int) -> None:
        ... # rest of the code

Is this correct?

[Tinche]

Yeah, that looks good, with the caveat you'll have to use x: int = field() to avoid a runtime error.

Unfortunately, type checkers don't really support checking validators defined like this. I just tried with Mypy and it didn't report an error with a different (wrong) signature. So for the time being, these annotations are more for documentation :(

[jacopoabramo]

I see, well good to know then. Thanks! Closing.