Functional Programming in Python

Chapter 2. Callables

The emphasis in functional programming is, somewhat tautologously, on calling functions. Python actually gives us several different ways to create functions, or at least something very function-like (i.e., that can be called). They are:

• Regular functions created with def and given a name at definition time

• Anonymous functions created with lambda

• Instances of classes that define a __call()__ method

• Closures returned by function factories

• Static methods of instances, either via the @staticmethod decorator or via the class __dict__

• Generator functions

This list is probably not exhaustive, but it gives a sense of the numerous slightly different ways one can create something callable. Of course, a plain method of a class instance is also a callable, but one generally uses those where the emphasis is on accessing and modifying mutable state.

Python is a multiple paradigm language, but it has an emphasis on object-oriented styles. When one defines a class, it is generally to generate instances meant as containers for data that change as one calls methods of the class. This style is in some ways opposite to a functional programming approach, which emphasizes immutability and pure functions.

Any method that accesses the state of an instance (in any degree) to determine what result to return is not a pure function. Of course, all the other types of callables we discuss also allow reliance on state in various ways. The author of this report has long pondered ...