Add object.__subclasshook__

[NeilGirdhar]

Consider:

from dataclasses import dataclass, is_dataclass
from abc import ABC
from typing import Any

class AwesomeDataclassInstance(ABC):
    def __init_subclass__(cls) -> None:
        super().__init_subclass__()
        dataclass(cls)

    @classmethod
    def __subclasshook__(cls, sub: Any, /) -> bool:
        return isinstance(sub, type) and is_dataclass(sub) or super().__subclasshook__(sub)

It will fail because __subclasshook__ is not found.

[AlexWaygood]

One the one hand, this is incorrect: neither abc.ABC nor abc.ABCMeta define __subclasshook__. The reason why the super().__subclasshook__ call works is that it's actually builtins.object that defines a __subclasshook__ method:

>>> object.__dict__["__subclasshook__"]
<method '__subclasshook__' of 'object' objects>
>>> object.__subclasshook__(int)
NotImplemented

On the other hand, calling __subclasshook__ makes no sense unless you're doing it on a class that has ABCMeta as the metaclass. So I'm not sure I'd want to add it to the stub for builtins.object. So maybe this is an acceptable "white lie".

I'm curious what other maintainers think on this. In any event, if we are going to do this, I think a comment in the stub would be warranted, since it isn't actually defined in the abc.ABC class at runtime.

[NeilGirdhar]

One the one hand, this is incorrect:

I'm also curious what the experts think. In my experience, it's better to be accurate (rather than the "white lie" that's here) since you don't know what someone might do. So, if you agree, I'll move the method to object.

On the other hand, why is this on object instead of on ABC? Isn't that just polluting object? Should we suggest that Python move this method to ABC?

[srittau]

In my opinion, we should stay as close to the implementation as possible, except if there's a really good reason. I believe we should add object.__subclasshook__.

[JelleZijlstra]

Agree, we should add object.__subclasshook__ in typeshed since it exists at runtime.

Possibly it shouldn't exist, but that's a decision to make on the CPython project. I don't understand the implementation of ABCs well enough to understand why the decision was made to add this method to object, but maybe there's a good reason.

[AlexWaygood]

Sounds good, let's move it to builtins.object then.

[NeilGirdhar]

Makes sense, done!

[github-actions]

According to mypy_primer, this change has no effect on the checked open source code. 🤖🎉