Error occure when using classonlymethod decorator

[RobertM15]

Bug report

from django.utils.decorators import classonlymethod


class Foo:
    @classonlymethod
    def bar(cls):
        pass

Error from mypy:

error: Argument 1 to "classonlymethod" has
incompatible type "Callable[[Foo], Any]"; expected "Callable[[Any, VarArg(Any), KwArg(Any)], Any]"  [arg-type]

What's wrong

Error occurred, but it seems to be more related to mypy itself. The problem is described here

I created this issue here, because I think the solution is very easy to fix it and what I understood from here there will not change how mypy treats subclasses of classmethod

Problem stub:
django-stubs/utils/decorators.pyi

class classonlymethod(classmethod): ...

How is that should be

Changing to alias solves the error

classonlymethod = classmethod

Let me know what do you think

System information

• OS:
• python version: 3.9.16
• django version: 3.2.18
• mypy version: 1.2.0
• django-stubs version: 4.2.0
• django-stubs-ext version: 4.2.0

[intgr]

I've experienced this issue myself as well. I'm using # type: ignore[arg-type] right now.

I agree that aliasing the types (classonlymethod = classmethod) should fix it. The downside is that type checkers won't know that classonlymethod methods aren't accessible on instances. But that's already the case right now.

Not sure if there could be other downsides?

Ideally this would be solved by adding mypy plugin logic, to teach mypy about instance behavior. But not a high priority IMO.

[adamchainz]

I've made #1494 to switch to an alias.