python typing signature (typing.Callable) for function with kwargs

Question

I heavily use python typing support from python 3.

Recently I was trying to pass a function as an argument and I do not find any help for using kwargs in typing.Callable signature.

Please check the code below and the comments.

import typing

# some function with singnature typing
def fn1_as_arg_with_kwargs(a: int, b: float) -> float:
    return a + b

# some function with singnature typing
def fn2_as_arg_with_kwargs(a: int, b: float) -> float:
    return a * b

# function that get callables as arg
# this works with typing
def function_executor(
        a: int, 
        b: float, 
        fn: typing.Callable[[int, float], float]):
    return fn(a, b)

# But what if I want to name my kwargs 
# (something like below which does not work)
# ... this will help me more complex scenarios 
# ... or am I expecting a lot from python3 ;)
def function_executor(
        a: int, 
        b: float, 
        fn: typing.Callable[["a": int, "b": float], float]):
    return fn(a=a, b=b)

Answer 1

You are probably looking for Callback protocols.

In short, when you want to express a callable with a complex signature, what you'll want to do is to create a custom Protocol that defines a __call__ method with the precise signature you want.

For example, in your case:

from typing import Protocol

# Or, if you want to support Python 3.7 and below, install the typing_extensions
# module via pip and do the below:
from typing_extensions import Protocol

class MyCallable(Protocol):
    def __call__(self, a: int, b: float) -> float: ...

def good(a: int, b: float) -> float: ...

def bad(x: int, y: float) -> float: ...


def function_executor(a: int, b: float, fn: MyCallable) -> float:
    return fn(a=a, b=b)

function_executor(1, 2.3, good)  # Ok!
function_executor(1, 2.3, bad)   # Errors

If you try type-checking this program using mypy, you'll get the following (admittedly cryptic) error on the last line:

Argument 3 to "function_executor" has incompatible type "Callable[[int, float], float]"; expected "MyCallable"

(Callback protocols are somewhat new, so hopefully the quality of the error messages will improve over time.)