[Leejohnston]

Chapter 14 — Functions: Reusable Code Blocks
Functions allow you to bundle code into reusable, organised pieces — the foundation of real software.

A function is a block of code you can run whenever you need it.

Example:

Code:

def greet():
    print("Hello from Python!")

Call it:

Code:

greet()

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14.1 Why Functions Matter

Functions help you:

• avoid repeating code 
• keep programs clean 
• break big problems into smaller parts 
• reuse logic anywhere 
• make code easier to read and maintain 

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14.2 Creating Your First Function

Code:

def welcome():
    print("Welcome to the Lumin Archive!")

Calling the function:

Code:

welcome()

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14.3 Functions With Parameters

Parameters are input values the function needs.

Code:

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

Call it:

Code:

greet("Mia")
greet("Johno")

Output:

Code:

Hello, Mia!
Hello, Johno!

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14.4 Multiple Parameters

Code:

def add(a, b):
    print(a + b)

Call:

Code:

add(3, 7)

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14.5 Functions That Return Values

Return sends a value BACK to the program.

Code:

def multiply(a, b):
    return a * b

result = multiply(4, 5)
print(result)

Output:

Code:

20

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14.6 return vs print

print() 
• shows text to the user 
• does NOT give data back to your program 

return 
• sends data back 
• allows reuse 
• allows storing values 

Example:

Code:

def square(n):
    return n * n

value = square(6)
print(value)

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14.7 Default Parameter Values

Code:

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

Code:

greet()          # Hello, friend!
greet("Lee")      # Hello, Lee!

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14.8 Keyword Arguments

You can specify parameters by name:

Code:

def power(base, exp):
    return base ** exp

print(power(exp=3, base=2))

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14.9 Scope — Local vs Global Variables

Local variables exist ONLY inside a function.

Code:

def test():
    x = 10
    print(x)

Global variables exist everywhere.

Code:

score = 0  # global

def add_point():
    global score
    score += 1

Avoid globals as much as possible — functions should be self-contained.

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14.10 Function Design — Best Practices

✔ clear name 
✔ does one job 
✔ short 
✔ returns a value if needed 
✔ parameters instead of globals 

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14.11 Mini Project — Area Calculator

Write functions:

• area_rectangle(width, height) 
• area_circle(radius) 

Ask user which shape they want and calculate the answer.

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14.12 Challenge — Bank Account Simulator

Write functions:

• deposit(balance, amount) 
• withdraw(balance, amount) 
• show_balance(balance) 

Rules:

• withdrawing too much gives an error 
• each function returns the new balance 
• track balance using variables 

Program flow example:

Code:

Balance: £100 
Deposit £20 → £120 
Withdraw £50 → £70 

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14.13 Chapter Summary

• functions organise code 
• parameters = inputs 
• return = output 
• functions improve readability 
• avoid global variables 
• functions are essential for all real programs 

Next:
Chapter 15 — Modules & Importing Code

This is where Python becomes *infinite* — you unlock tools written by millions of developers.

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Written and Compiled by Lee Johnston — Founder of The Lumin Archive