Metaclass methods incorrectly identified as classmethods

[jamesdow21]

ruff incorrectly believes that the __call__ method in a metaclass should be a classmethod and gives error N804 that the first argument should be named "cls" instead of "self", but only for immediate subclasses of type or ABCMeta (and possibly others, those were the only two I've tried)

saving the following in a file named "meta_call.py" and checking with ruff gives an error only on the __call__ definitions of BaseMeta and ABCBaseMeta

from abc import ABCMeta
from typing import Any

class BaseMeta(type):
    def __call__(self, *args: Any, **kwds: Any) -> Any:
        return super().__call__(*args, **kwds)

class SubMeta(BaseMeta):
    def __call__(self, *args: Any, **kwds: Any) -> Any:
        return super().__call__(*args, **kwds)

class ABCBaseMeta(ABCMeta):
    def __call__(self, *args: Any, **kwds: Any) -> Any:
        return super().__call__(*args, **kwds)

class SubABCMeta(ABCBaseMeta):
    def __call__(self, *args: Any, **kwds: Any) -> Any:
        return super().__call__(*args, **kwds)

class AMeta(type):
    pass

class BMeta(AMeta):
    def __call__(self, *args: Any, **kwds: Any) -> Any:
        return super().__call__(*args, **kwds)

class CMeta(BMeta):
    def __call__(self, *args: Any, **kwds: Any) -> Any:
        return super().__call__(*args, **kwds)

command: ruff check --select N meta_call.py version: 0.0.280 default settings (i.e. just running from ruff freshly installed into pipx standalone environment)

ruff's output

meta_call_err.py:6:18: N804 First argument of a class method should be named `cls`
meta_call_err.py:16:18: N804 First argument of a class method should be named `cls`
Found 2 errors.

ABCMeta itself inherits directly from type and doesn't do anything to modify the definition of __call__

In typeshed at least, the parameter name for type.__call__ is self

[jamesdow21]

I know the choice of cls or self in a metaclass can be a touch confusing, because an instance (expected to be named "self" in an instance method) of a metaclass is a class (which is normally expected to be assigned to "cls")

I wasn't sure if this was some special casing for metaclasses in the naming rules, but if so, it doesn't seem to be accounting for the possibility of sub-metaclasses

[jamesdow21]

After a bit more testing, this is actually much more general for any method on a metaclass derived directly from either type or ABCMeta (which as far as I can tell are the only two metaclasses available from the standard library)

from abc import ABCMeta

class A(type):
    def foo(self) -> None:
        print("I am an instance method of metaclass A")

class B(A):
    def bar(self) -> None:
        print("I am an instance method of metaclass B")

class C(ABCMeta):
    def baz(self) -> None:
        print("I am an instance method of metaclass C")

ruff reports N804 errors on A.foo and C.baz methods, but not B.bar

[charliermarsh]

I think it is special-cased somewhere, and we likely did that to follow some other tool's conventions, probably pep8-naming which has the same behavior as Ruff on the snippet you just shared:

❯ flake8 foo.py
foo.py:3:1: E302 expected 2 blank lines, found 1
foo.py:4:14: N804 first argument of a classmethod should be named 'cls'
foo.py:14:14: N804 first argument of a classmethod should be named 'cls'

[jamesdow21]

I don't know rust in particular, but if I understand correctly I believe this snippet is where that special casing is done

https://github.com/astral-sh/ruff/blob/5dbb4dd8235e389ae299d57fb23e36bcc9995c66/crates/ruff_python_semantic/src/analyze/function_type.rs#L46-L52

Looks like it is indeed special casing exactly "type" or "abc.ABCMeta" as the direct parent class