I read PEP 612 (Parameter Specification Variables)

This is the second installment of the PEP series, which I heard for the first time when I was scanning the README of Microsoft's checker for Python pyright. This time, I will write about PEP 612 (Parameter Specification Variables).

Overview

• Very difficult to write a type when trying to write a general purpose decorator
• ** I want a way to represent a function ** that has the same arguments as the specified function **

approach

• Resolved by adding a type that represents the argument ParameterSpecification

Add ParameterSpecification as the type that represents the argument. The ParameterSpecification can be used in conjunction with the Callable to behave like the generics of a callable object. It is easy to understand if you think of it as an argument version of TypeVar.

Example

In the example below, the argument of ʻadd_logging ()is of typeCallable [Ps, R], and the return value is also of type Callable [Ps, R]`. That is, it is a decorator that returns a function with the same interface as the function specified as an argument.

Therefore, foo () decorated with @ add_logging remains of type (x: int, y: str)-> int.

from typing import Callable, ParameterSpecification, TypeVar

Ps = ParameterSpecification("Ps")
R = TypeVar("R")

def add_logging(f: Callable[Ps, R]) -> Callable[Ps, R]:
    def inner(*args: Ps.args, **kwargs: Ps.kwargs) -> R:
        log_to_database()
        return f(*args, **kwargs)

    return inner


@add_logging
def foo(x: int, y: str) -> int:
    return x + 7

It's difficult to type these decorators if the ParameterSpecification type isn't introduced.

Impressions

• Fortunately I didn't notice it because I hadn't typed the decorator before, but it certainly doesn't have the proper type without the ParameterSpecification.
• The name ParameterSpecification is a bit long, but I couldn't think of any alternatives ...
• It's a pity that not all of the argument-tampering type decorators are resolved as this PEP does not handle them.
• Since it is Draft yet, adoption has not been decided yet