Python typing: pylance doesn't show input types

Question

I'm trying to do "wrapper" class for any callable. Here is minimal example:

from typing import Any, TypeVar, Generic, Callable

ReturnType = TypeVar("ReturnType", bound=Any)
CallableType = Callable[..., ReturnT]


class Wrapper(Generic[ReturnType]):
    def __init__(self, callable_: CallableType[ReturnType]) -> None:
        self.callable_ = callable_

    def __call__(self, *args: Any, **kwargs: Any) -> ReturnType:
        return self.callable_(*args, **kwargs)


class Action:
    def __init__(self, title: str) -> None:
        """docstring"""
        self.title = title


wrapped = Wrapper(Action)

action = wrapped("title")

Pylance can understand, that action type is Action, but I can't see __init__ docsting and needed arguments:

Desired behaviour:

What do I do wrong and how can I fix it?

Answer 1

You've annotated your __call__ with Any. Pyright doesn't restrict these Any to more narrow types just based on your usage. You'll need to use a ParamSpec to capture the parameters of the callable you accept as the argument (similar how you capture the return type with a TypeVar).

from typing import Any, TypeVar, ParamSpec, Generic, Callable

Params = ParamSpec("Params")
ReturnType = TypeVar("ReturnType")


class Wrapper(Generic[Params, ReturnType]):
    def __init__(self, callable_: Callable[Params, ReturnType]) -> None:
        self.callable_ = callable_

    def __call__(self, *args: Params.args, **kwargs: Params.kwargs) -> ReturnType:
        return self.callable_(*args, **kwargs)


class Action:
    def __init__(self, title: str) -> None:
        """docstring"""
        self.title = title


wrapped = Wrapper(Action)

action = wrapped("title")

Note1: If you're using python<3.10 you'll need to import ParamSpec from typing_extensions.

Note2: You don't need to pass a bound=Any to the TypeVar as that's (more or less) the default anyway. Passing bound=Any doesn't change anything nevertheless.