Shared via mypy Playground

main.py

from typing import overload, Literal


class Foo:
    @overload
    def some_method(self, bar: str = '', *, flag: Literal[True]) -> int:
        ...

    @overload
    def some_method(self, bar: str = '', *, flag: Literal[False]) -> list[int]:
        ...

    def some_method(self, bar: str = '', flag: bool = False) -> int | list[int]:
        if flag:
            return len(bar)
        else:
            return [len(bar)]


f = Foo()

f.some_method()