Defining Functions

As programs grow, you may need to use the same code many times.

Writing the same code again and again takes time and makes programs harder to manage.

Functions solve this problem.

A function lets you place code inside a reusable block. You can run that block whenever you need it.

What Is a Function?

A function is a group of code with a name.

That code is saved and can be used later.

Think of a function like a machine button:

• Press the button
• The machine does its job

When you call a function, Python runs the code inside it.

Why Functions Matter

Functions help you:

• Reuse code
• Keep programs clean
• Save time
• Organize large programs
• Fix code in one place

Real Example Without a Function

print("Hello")
print("Hello")
print("Hello")

This works, but repeating code is not the best habit.

Better Example With a Function

def greet():
    print("Hello")

greet()
greet()
greet()

Output

Hello
Hello
Hello

Understanding def

The word def is a Python keyword.

It means you are defining (creating) a function.

def greet():

Let’s break it down:

• def tells Python a function is starting
• greet is the function name
• () is where extra values can go later
• : starts the code block

The Code Inside the Function

Indented lines belong to the function.

def greet():
    print("Hello")

The spaces before print() are important.

They tell Python this line is part of the function.

Important: Creating Is Not Running

This code creates a function, but does not run it yet.

def greet():
    print("Hello")

Nothing happens until you call it.

Calling a Function

To run the function, type its name followed by brackets.

greet()

This tells Python to run the saved code.

Another Example

def welcome():
    print("Welcome to Python")

welcome()

Output

Welcome to Python

Common Beginner Mistakes

Forgetting the Brackets

Wrong:

greet

Correct:

greet()

Forgetting Indentation

Wrong:

def greet():
print("Hello")

Correct:

def greet():
    print("Hello")

Code Along

def show_name():
    print("Sam")

show_name()

Output

Sam

Mini Challenge

Build a greeting function.

Steps:

• Create a function called hello
• Inside the function, print:
  Hello Student
• Call the function two times

Expected output:

Hello Student
Hello Student

Real World Use Case

Programs use functions for calculations, login checks, emails, reports, games, and many repeated tasks.

Quiz

1. What is a function?
2. Which keyword creates a function?
3. Does a function run when it is created?
4. How do you run a function?

Assignment

Create a function called bye that prints Goodbye, then call it three times.

Summary

You learned that functions are reusable blocks of code, how to create them with def, and how to run them by calling the function name.