Functions in Python can be nested inside other functions.

A function defined inside a function is visible only inside that function.

This is useful to create utilities that are useful to a function, but not useful outside of it.

You might ask: why should I be “hiding” this function, if it does not harm?

One, because it’s always best to hide functionality that’s local to a function, and not useful elsewhere.

Also, because we can make use of closures (more on this later).

Here is an example:

def talk(phrase):
    def say(word):
        print(word)

    words = phrase.split(' ')
    for word in words:
        say(word)

talk('I am going to buy the milk')

If you want to access a variable defined in the outer function from the inner function, you first need to declare it as nonlocal:

def count():
    count = 0

    def increment():
        nonlocal count
        count = count + 1
        print(count)

    increment()

count()

This is useful especially with closures, as we’ll see later.