For a an complex API client I have build an decorator which injects a http session into the method inside a class.

def do_auth(
    fn: Callable[Concatenate['ApiClient', _HttpSession, P], RetType],
) -> Callable[Concatenate['ApiClient', str | None, P], RetType]:

The function inside the decorator looks ruffly like this:

    @functools.wraps(fn)
    def wrapper(self: 'ApiClient', client_id: str | None = None, *args: P.args, **kwargs: P.kwargs) -> RetType:

The typing of the decorator is slightly wrong since the wrapper function defines a default value of None. But I haven't found a way to tell my type checker that.

The mypy docs say that Protocol classes are the correct way to type this, but I wasn't able with the self parameter. Can someone point me into the correct direction?