I’m trying to figure out how to write a decorator that passes strict type checks as well as preserve type information about the original arguments of a function.
What makes this case tricky, is that the decorator must drop the first argument of the function it is wrapping.

I’ve been able to get past most of the typing/mypy errors using Concatenate, ParamSpec etc…, but now I’m stuck.

Here is what I have so far:

from functools import wraps
from typing import Any, Callable, Concatenate, ParamSpec, TypeVar

P = ParamSpec('P')
R = TypeVar('R')

def drop_first_arg(f: Callable[Concatenate[Any, P], R]) -> Callable[P, R]:
    @wraps(f)
    def wrapper(*args: P.args, **kwargs: P.kwargs) -> R:
        # Error on "return" line:
        # MyPy(4.10.0): error: Argument 1 has incompatible type "*tuple[object, ...]";
        #       expected "P.args"  [arg-type]
        # Pylance: Arguments for ParamSpec "P@drop_first_arg" are missing
        #          (reportCallIssue)
        return f(*args[1:], **kwargs)  # `[1:]` effectively drops the first argument
    return wrapper

When you pass the above through mypy --strict, then you’ll see the following error message:

error: Argument 1 has incompatible type "*tuple[object, ...]"; expected "P.args"  [arg-type]

So essentially this all boils down to the question: How can I write a drop_first_argument decorator that will also pass all of mypy --strict‘s checks?