Python: What is ParamSpec and Concatenate?

1from typing import ParamSpec, TypeVar, Callable
2from typing import Concatenate
3
4P = ParamSpec("P")
5R = TypeVar("R")
6
7# Decorator that preserves signature
8def log_calls(func: Callable[P, R]) -> Callable[P, R]:
9    def wrapper(*args: P.args, **kwargs: P.kwargs) -> R:
10        print(f"Calling {func.__name__}")
11        return func(*args, **kwargs)
12    return wrapper
13
14@log_calls
15def add(a: int, b: int) -> int:
16    return a + b
17
18add(1, 2)  # Type checker knows this is (int, int) -> int
19
20# Concatenate — add parameters
21def with_request(
22    func: Callable[Concatenate[Request, P], R]
23) -> Callable[P, R]:
24    def wrapper(*args: P.args, **kwargs: P.kwargs) -> R:
25        request = get_current_request()
26        return func(request, *args, **kwargs)
27    return wrapper
28
29@with_request
30def get_user(request: Request, user_id: int) -> User:
31    return User.objects.get(id=user_id)
32
33get_user(42)  # Request is injected, type checker knows!