Writing higher-order functions

We can identify three varieties of higher-order function; they are as follows:

• Functions that accept a function as one of their arguments.
• Functions that return a function. A Callable class is a common example of this. A function that returns a generator expression can be thought of as a higher-order function.
• Functions that accept and return a function. The functools.partial() function is a common example of this. We'll save this for Chapter 10, The Functools Module. A decorator is different; we'll save this for Chapter 11, Decorator Design Techniques.

We'll expand on these simple patterns using a higher-order function to also transform the structure of the data. We can do several common transformations, such as the following:

• Wrap objects to create more complex objects
• Unwrap complex objects into their components
• Flatten a structure
• Structure a flat sequence

A Callable class object is a commonly used example of a function that returns a callable object. We'll look...