Type annotation for function returning a lambda

Question

I’m very fond of the idea of using type annotations in Python. I know how to do such like in this simple example:

def foo(bar : int, lol : int) -> int:
     return bar*lol

But I have no clue of how to do so when my function is going to return a lambda:

def line(slope : float, b : float) -> lambda:
     return lambda x: slope*x + b

This example generates an error, and I wonder what’s the keyword I’m supposed to use for this return type.

Perhaps this doesn’t seem so useful but I want to keep my code consistently annotated and being unable to do so with these type of functions is really bothering me.

Thanks in advance.

Answer 1

You can use Callable to type hint the return:

from typing import Callable

def line(slope: float, b : float) -> Callable[[float], float]:
     return lambda x: slope*x + b