Rule request: Warn whenever any default value is provided in __post_init__

[NeilGirdhar]

Default values should not be given in the arguments of __post_init__. They should only be provided as default values for class-level init-only member variables.

from dataclasses import InitVar, dataclass


@dataclass
class C:
    x: InitVar[int] = 1

    def __post_init__(self, x: int = 4) -> None:
        print(x)  # noqa: T201


c = C()  # Prints 1!  4 is ignored.

The above text could be used for the Ruff rule warning message.

[dhruvmanila]

Interesting, thanks for raising this rule request. This seems like a reasonable rule to implement, most likely in a RUF category. What do you think? @AlexWaygood

[dhruvmanila]

Although, I couldn't really find any usages of such pattern: https://grep.app/search?q=def%20__post_init__%5C(self,%5B%5E%3D%5Cn%5D+%3D&regexp=true&filter%5Blang%5D%5B0%5D=Python

[NeilGirdhar]

Although, I couldn't really find any usages of such pattern

Fair enough. It came up when I was working on my own code. I had accidentally added some parameter defaults, and then I though to check what happens.

That said, if this pattern is that rare, then I agree that there are more important issues to be added to Ruff and we can close this.

[AlexWaygood]

I agree this is a reasonable thing to have a rule for. There's no chance of false positives, and I think I remember hitting this footgun myself at one point in a previous project. The RUF category sounds good to me.