Optinal in Callable with self

[ninanomenon]

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?

[JelleZijlstra]

The protocol should look somewhat like this (untested):

class AuthWrapper[**P, R]:
    def __call__(self, client: ApiClient, client_id: str | None = None, /, *args: P.args, **kwargs: P.kwargs) -> R: ...

(The "self" here refers to the callable itself, not the client object.)

[cleonaia]

I think the tricky part here is that Callable can express the parameter types, but it can't fully describe the default value of a parameter.

Since client_id is optional, I'd try defining the wrapper's signature explicitly with a Protocol, keeping self as the first parameter:

from typing import Protocol, TypeVar, ParamSpec

P = ParamSpec("P")
R = TypeVar("R")

class WrappedMethod(Protocol[P, R]):
    def __call__(
        self,
        client_id: str | None = None,
        *args: P.args,
        **kwargs: P.kwargs,
    ) -> R: ...

The exact typing depends on whether the decorated function is a bound method or an unbound method, since self is handled differently in those cases. I'd also check the behaviour with both mypy and pyright, as support for these signatures can differ.

The important distinction is that str | None describes the accepted values, while = None describes the default. Callable doesn't preserve that default in its type.