Closures in Python

Closures are a powerful and flexible way to create new functions out of existing functions. You can think of them as being function factories, that can create new functions according to a template and one or more parameters.

In this article we will look at how to use a closure to compose two functions, as a simple illustration of why and how we use closures.

This article first appeared on pythoninformer.com. The topic is covered in more detail in my e-book Functional Programming in Python.

Composing functions

Composing two functions simply means applying one function to the result of another. For example, consider these two standard Python functions:

str(x)
len(x)

str(x)converts any value x to a string. len(x)returns the length of any sequence x. The composition of these two functions is

Inner functions

An inner function is a function that is defined within the scope of another function, like this:

def add_3(v):
 n = 3
 def f(x):
  return x + n
 return f(v)
print(add_3(6)) # Prints 9

In this case our outer function add_3 adds 3 to the value of xand returns the result.