Functions

A function in Python is a named, reusable block of code that performs a specific task, optionally takes inputs (arguments) and returns a result, so you can run the same logic many times without repeating it.

Part of the free Python course at LearnCodingFast — hands-on lessons with examples you run in your browser, plus practice exercises and a quick quiz.

Learn to organize, reuse, and simplify your code with Python functions.

What You'll Learn in This Lesson

1. What Are Functions?

A function is a reusable block of code that performs a specific task. Instead of writing the same code over and over, you write it once in a function and call it whenever you need it.

Why Use Functions?

• Reusability — Write once, use many times
• Organization — Break complex programs into smaller pieces
• Easier debugging — Fix bugs in one place, not everywhere
• Readability — Code is easier to understand

2. Defining a Function

Part

Meaning

def

Keyword that starts a function definition

function_name

The name you give your function (use snake_case)

()

Parentheses for parameters (empty if none)

:

Colon to start the function body

Indented code

The code that runs when you call the function

3. Parameters — Giving Functions Inputs

Parameters are variables listed inside the parentheses when defining a function. They allow you to pass information into the function.

4. Return Values — Getting Results Back

The return keyword sends a value back to the code that called the function. This lets you use the result!

5. Default Parameter Values

You can give parameters default values . If no argument is passed, the default is used:

6. Positional vs Keyword Arguments

When calling a function, you can pass arguments in two ways:

Type

Example

Description

Positional

func("Alice", 25)

Arguments matched by position/order

Keyword

func(name="Alice", age=25)

Arguments matched by name (any order)

7. Returning Multiple Values

Python functions can return multiple values at once! They're returned as a tuple.

8. Variable Scope — Local vs Global

Scope determines where a variable can be accessed:

Scope

Where Defined

Where Accessible

Local

Inside a function

Only inside that function

Global

Outside all functions

Everywhere in the file

9. Docstrings — Documenting Your Functions

A docstring is a special string that describes what your function does. Put it right after the def line:

10. Common Mistakes to Avoid

Mistake

Wrong

Correct

Forgetting parentheses

greet

greet()

Forgetting colon

def greet()

def greet():

Missing indentation

def greet(): print("Hi")

Using print instead of return

print(a + b)

return a + b

Wrong argument count

add(5)

add(5, 3)

11. Practical Examples

Example 1: Temperature Converter

Example 2: Grade Calculator

Example 3: Simple Calculator

Summary: Quick Reference

Concept

Syntax

Define function

def name():

With parameter

def name(param):

def greet(name):

Return value

return value

Default param

param="default"

def greet(name="Guest"):

Keyword arg

param=value

greet(name="Alice")

Multiple returns

return a, b

x, y = func()

Docstring

"""text"""

"""Calculate area."""

Lesson 6 done — you can now write reusable code like a pro!

Functions are the single most important concept in all of programming. You now know how to define them, pass data in, get results out, set defaults, and return multiple values.

🚀 Up next: Lists — learn to store and manage collections of data, the backbone of most real programs.

Practice quiz

Continue this course

• Previous: Loops - For and While
• Next: Lists and Collections