a classonlymethod decorator

[KotlinIsland]

class A:
    @classmethod
    def a(cls): ...
    @classonlymethod
    def b(cls): ...
A.a()  # ok
A.b()  # ok
A().a()  # ok
A().b()  # not ok (😞)

The reasoning is that the classmethod decorator is often used for alternative constructors, so using it on instances would be a mistaken usage.

implementation

We could attempt a vanilla implementation using descriptors, otherwise a simple alias

if not TYPE_CHECKING:
    classonlymethod = classmethod
else:
    class classonlymethod(classmethod):  #  or something
        ...

[DetachHead]

classonlymethod can be implemented entirely using descriptors, and it will work in basedpyright with no additional changes/special casing. for it to work in basedmypy tho, python/mypy#17050 will need to be fixed

i think staticonlymethod is kinda redundant because you can achieve the same effect by just making it a function outside of the class