111 – Type hints that refer to functions

When dealing with higher-order functions (functions that accept functions as arguments or that return functions), you can use collections.abc.Callable to add a type hint to the function that is an argument/the return value.

Callable takes two values:

1. the list of types of the arguments of the callable; and

2. the return type of the callable.

For example, Callable[[str, bool], None] is the type that refers to functions that accept a string and a Boolean value and return None.