how_to_type_this

test.py

import random
from functools import wraps
from typing import TYPE_CHECKING, Any, Callable, Union

from typing_extensions import Concatenate, ParamSpec

P = ParamSpec('P')


def maybe_inject_int(method: Callable[Concatenate['DBHandler', int, P], Any]) -> Union[
        Callable[Concatenate['DBHandler', P], Any],
        Callable[Concatenate['DBHandler', int, P], Any],
]:
    """Wraps the method of DBHandler to maybe inject a positional int argument
    """
    @wraps(method)
    def _impl(self: 'DBHandler', *args: Any, **kwargs: Any) -> Union[
            Callable[P, Any],
            Callable[Concatenate[int, P], Any],
    ]:
        if isinstance(args[0], int):  # first argument is the int
            return method(self, *args, **kwargs)
        # else we need to inject the argument
        new_int = random.randint(1, 99)
        result = method(self, new_int, *args, **kwargs)
        return result
    return _impl


class DBHandler:

    def __init__(self) -> None:
        self.id = 1

    @maybe_inject_int
    def foo(self, number: int, name: str) -> str:
        return f'{self.id=} {name=} {number=}'


db = DBHandler()

if TYPE_CHECKING:
    reveal_type(db.foo)
print(db.foo(5, 'John'))
print(db.foo('Mary'))