autodoc: fix detection of class methods implemented by extension modules

[picnixz]

Closes #13188.

cc @skirpichev

@AA-Turner do we have tests for this? I tested it locally but I'm not sure we're building a C extension module in Sphinx to test this.

[skirpichev]

Works.

[picnixz]

Btw, I'm not sure if this is something CPython wants to improve by adding some inspect.isclassmethod since isinstance(m, staticmethod) works both for @staticmethod in Python and METH_STATIC in C, but isinstance(m, classmethod) is only for @classmethod in Python (we need isinstance(m, types.ClassMethodDescriptorType) for those in C).

[skirpichev]

I'm not sure we're building a C extension module in Sphinx to test this.

Can't you use CPython builtin classes (e.g. int) to test this?

BTW, there is no "official" way to describe signatures for extensions. The gmp module (as many CPython builtins) uses funny notation in __text_signature__, the gmpy2 module instead just uses PEP 7 -style "signature lines" in docstrings (currently classmethods displayed here incorrectly too: https://gmpy2.readthedocs.io/en/latest/mpz.html#gmpy2.mpz.from_bytes). The inspect module doesn't work in that case.

[picnixz]

I can use built-in classes but I wanted some complex inheritance just in case. I guess I can write some tests with what I have already in hand though.

[skirpichev]

I can use built-in classes but I wanted some complex inheritance just in case.

IIRC, multiple inheritance does not work for statically defined types.

[picnixz]

Oh I see. Though, I thought about inheriting (in pure Python) from my custom built-in class just to check that we correctly pick up something written in C as well. What I didn't want to do is use a built-in whose signature rendering may change across versions for various reasons. It makes tests harder to maintain (something that usually happens for enums in general...)

[AA-Turner]

I agree with Sergey that using e.g. int.from_bytes or a similar builtin classmethod descriptor would be good. We don't currently have any test infrastructure to build a C extension module.

A

[AA-Turner]

Does this still need to recurse? I'd prefer being explicit here (even if duplicative):

Suggested change

	return is_builtin_classmethod(meth)
	return is_classmethod_descriptor(obj, cls, name) or (
	isbuiltin(obj)
	and getattr(obj, '__self__', None) is not None
	and isclass(obj.__self__)
	)