Functional Programming in Python

Chapter 4. Higher-Order Functions

In the last chapter we saw an iterator algebra that builds on the itertools module. In some ways, higher-order functions (often abbreviated as “HOFs”) provide similar building blocks to express complex concepts by combining simpler functions into new functions. In general, a higher-order function is simply a function that takes one or more functions as arguments and/or produces a function as a result. Many interesting abstractions are available here. They allow chaining and combining higher-order functions in a manner analogous to how we can combine functions in itertools to produce new iterables.

A few useful higher-order functions are contained in the functools module, and a few others are built-ins. It is common the think of map(), filter(), and functools.reduce() as the most basic building blocks of higher-order functions, and most functional programming languages use these functions as their primitives (occasionally under other names). Almost as basic as map/filter/reduce as a building block is currying. In Python, currying is spelled as partial(), and is contained in the functools module—this is a function that will take another function, along with zero or more arguments to pre-fill, and return a function of fewer arguments that operates as the input function would when those arguments are passed to it.

The built-in functions map() and filter() are equivalent to comprehensions—especially now that generator comprehensions are available—and ...