How to define signature of a callable argument?

Question

Let's say I have a function that accepts a callable

def foo(bar):
    print(bar(1, 2))

What is the best way to declare that bar(..) accepts two ints and returns a str? Is there a more well-defined way than just documentation?

Answer 1

The Callable type from typing lets you specify both the argument types and the return type of a callable value.

from typing import Callable

def foo(bar: Callable[[int,int],str]):
    print(bar(1, 2))

Note that since Python is dynamically typed, a type hint is, to some extent, just documentation. Third-party tools like mypy , though, can be used for static type analysis to help ensure that your code doesn't pass a value of the wrong type.

Answer 2

Python is dynamically typed which means the interpreter type checks as the code runs, and the type can change over time.

PEP 484 introduced type hints. You can do this by the following syntax:

def bar( a: int, b: int) -> str:
    return 

def foo(bar: callable) -> None:
    print(bar(1, 2))