Functions in Python

Teach Python a new trick once, then reuse it forever

Big Idea: A function is a named block of code that does one job. You define it once with def, then call it whenever you need it — instead of copying the same lines again and again.

By the end you will be able to:

define a function with def and call it,
send information in with parameters and arguments,
send a result back with return,
tell a local variable from a global one (scope),
explain why functions make code shorter and clearer.

1. What is a function?

A function is like a little machine. You give it something, it does its job, and it can hand something back. A juicer is a good picture: you put in oranges (the input), it squeezes (the work), and out comes juice (the output). You built the juicer once, but you can use it every morning.

In code, a function lets you package a set of steps under one name. Then you run all those steps just by writing the name — this is called calling the function.

2. Defining a function with def

You create a function with the keyword def, a name, round brackets (), and a colon :. The steps inside are indented (usually 4 spaces).

def greet():
    print("Hello!")
    print("Welcome to Python.")

greet()
greet()

Output:

Hello!
Welcome to Python.
Hello!
Welcome to Python.

Notice: writing def greet(): only defines the function — nothing prints yet. The code runs only when you call it with greet(). We called it twice, so the message appears twice.

Functions in Python