# Functional Programming HOWTO — The functools module

> The functools module contains some higher-order functions. A higher-order function takes one or more functions as input and returns a new function. The most useful tool in this module is the functools.partial function. For programs written in a functional style, you'll sometimes want to construct va

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The functools module contains some higher-order functions. A higher-order function takes one or more functions as input and returns a new function. The most useful tool in this module is the functools.partial function.

For programs written in a functional style, you'll sometimes want to construct variants of existing functions that have some of the parameters filled in. Consider a Python function f(a, b, c); you may wish to create a new function g(b, c) that's equivalent to f(1, b, c); you're filling in a value for one of f()'s parameters. This is called "partial function application".

The constructor for ~functools.partial takes the arguments (function, arg1, arg2, ..., kwarg1=value1, kwarg2=value2). The resulting object is callable, so you can just call it to invoke function with the filled-in arguments.

Here's a small but realistic example

functools.reduce(func, iter, [initial_value]) cumulatively performs an operation on all the iterable's elements and, therefore, can't be applied to infinite iterables. func must be a function that takes two elements and returns a single value. functools.reduce takes the first two elements A and B returned by the iterator and calculates func(A, B). It then requests the third element, C, calculates func(func(A, B), C), combines this result with the fourth element returned, and continues until the iterable is exhausted. If the iterable returns no values at all, a TypeError exception is raised. If the initial value is supplied, it's used as a starting point and func(initial_value, A) is the first calculation.

If you use operator.add with functools.reduce, you'll add up all the elements of the iterable. This case is so common that there's a special built-in called sum to compute it

For many uses of functools.reduce, though, it can be clearer to just write the obvious for loop

import functools # Instead of: product = functools.reduce(operator.mul, [1, 2, 3], 1)

# You can write: product = 1 for i in [1, 2, 3]: product = i

A related function is itertools.accumulate(iterable, func=operator.add) . It performs the same calculation, but instead of returning only the final result, ~itertools.accumulate returns an iterator that also yields each partial result

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