Python Basics · Functions · Lesson 11 of 36

Defining Functions

Concept

Defining Functions

Functions are defined with def, and Python uses indentation to mark what is inside the function body.

A function without an explicit return statement returns None automatically, the same way JavaScript functions return undefined by default.

By the end of this lesson

Explain the core idea behind Defining Functions.
Predict output before running a change.
Test one realistic and one unusual input.
Use the result to make the next decision.

How to study this page

1. Read one concept.
2. Change the example.
3. Run, compare, and explain.
4. Complete the challenge below.

Syntax

def function_name(parameter):
    # code
    return value

ExampleRunnable

def greet(name):
    return f"Hello, {name}!"

print(greet("Ibrohim"))

Try it Yourself »

Self-check before continuing

Without looking at the example, describe what changes when you modify one input in Defining Functions. Then reopen the editor and prove your explanation with a small test.

You are ready to continue when you can predict, test, and explain the result.

Your turn

Write a function square(n) that returns n squared.

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Console

Output will appear here...