Module 4 · Functions · Lesson 3 of 4

Where functions come from

You've already been calling functions — print(), len(), input(). So where do functions actually come from? There are three sources, and the lovely part is that you call all of them the same way: a name, then parentheses.

Three sources, one way to call

built into Python

print() len() input()

from a module

math.sqrt() random.randint()

written by you

def greet(): ...

→

📞

call it!

Built in. A core set comes with Python and is always ready — print, len, input, sum, and more.
From a module. Python ships with a huge standard library; you import a module like math or random to unlock its functions.
Written by you. When nothing existing fits, you define your own with def — exactly what we'll do next.

Run it yourselfruns in your browser · Python

main.py

Built-in, imported, or your own — once a function exists, calling it looks identical. That consistency is what makes Python feel so approachable. Time to write one yourself.