We can define higher-order functions as instances of the Callable class. This builds on the idea of writing generator functions; we'll write callables because we need statement features of Python. In addition to using statements, we can also apply a static configuration when creating the higher-order function.

What's important about a Callable class definition is that the class object, created by the class statement, defines essentially a function that emits a function. Commonly, we'll use a callable object to create a composite function that combines two other functions into something relatively complex.

To emphasize this, consider the following class:

from collections.abc import Callable
class NullAware(Callable):
    def __init__(self, some_func):
        self.some_func= some_func
    def __call__(self, arg):
        return None if arg is None else self.some_func(arg)

This class creates a function named NullAware() that is a higher-order function...