>>> Python for Beginners Chapter 10 of 10

Chapter 10 · Reusing Code

Functions

As programs grow, you find yourself writing the same thing again and again. Functions let you write a block once, give it a name, and reuse it forever - the foundation of all real software.

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Real-world analogy

A function is a recipe. You write it down once ("how to make bread"), give it a name, and from then on you just say "make bread" instead of listing every step again. You can even hand it different ingredients each time.

Writing your own function

Use def, a name, brackets, and a colon. The indented block is the function's body:

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

greet()
greet()
Hello! Welcome to Python. Hello! Welcome to Python.

Defining a function doesn't run it — greet() is what actually runs it. Here we called it twice without repeating the code.

Giving a function information (parameters)

Values in the brackets let a function work on different data each time:

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

greet("Grace")
greet("Godfrey")
Hello, Grace! Hello, Godfrey!

Getting an answer back (return)

Functions can hand back a result with return, which you can store and use:

def add(a, b):
    return a + b

total = add(10, 5)
print(total)
print(add(100, 250))
15 350

The difference: print just shows something; return gives a value back to the program so you can keep working with it.

Putting it all together

def area_of_room(length, width):
    return length * width

rooms = [(4, 3), (5, 5), (2, 6)]
for length, width in rooms:
    print(f"A {length}x{width} room is {area_of_room(length, width)} square metres")
A 4x3 room is 12 square metres A 5x5 room is 25 square metres A 2x6 room is 12 square metres
✍️ Try it yourself

Write a function double(n) that returns a number times two. Call it with a few different numbers and print the results.

🎉 You've reached the end of the beginner course

Look how far you've come. You can now store data, make decisions, repeat work, hold collections, and package logic into reusable functions — that's the real core of programming, and it's the same in every language you'll ever learn.

What next? Keep the playground open and build small things: a tip calculator, a quiz, a to-do list. Building beats reading every time. When you're ready for the web, our PHP OOP course takes these same ideas into real websites with databases and login.

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