singledispatchmethod-function / E1520

Message emitted:

singledispatchmethod decorator should not be used with functions, use singledispatch instead.

Description:

singledispatchmethod should decorate class/instance methods and not functions. Use singledispatch for those cases.

Problematic code:

from functools import singledispatchmethod


@singledispatchmethod  # [singledispatchmethod-function]
def convert_position(position):
    print(position)


@convert_position.register  # [singledispatchmethod-function]
def _(position: str) -> tuple:
    position_a, position_b = position.split(",")
    return (int(position_a), int(position_b))


@convert_position.register  # [singledispatchmethod-function]
def _(position: tuple) -> str:
    return f"{position[0]},{position[1]}"

Correct code:

from functools import singledispatchmethod


class Board:
    @singledispatchmethod
    def convert_position(cls, position):
        pass

    @singledispatchmethod
    @classmethod
    def _(cls, position: str) -> tuple:
        position_a, position_b = position.split(",")
        return (int(position_a), int(position_b))

    @singledispatchmethod
    @classmethod
    def _(cls, position: tuple) -> str:
        return f"{position[0]},{position[1]}"

Created by the stdlib checker.