Lesson 5: Code Reuse with Functions (Introduction)

Functions are named, self-contained blocks of organized, reusable code designed to perform a single, related action. They are the fundamental tool for achieving modularity in programming, allowing complex systems to be broken down into smaller, manageable units.

1. The Power of Code Reuse

The defining benefit of functions is the ability to write a piece of logic once and execute it hundreds of times without copying and pasting. This reduces errors, improves efficiency, and makes large applications easier to maintain and scale. This principle is often referred to as DRY (Don't Repeat Yourself).

This lesson focuses on mastering the structure of defining a function, passing input information (arguments), and managing output results. We will use functions to abstract complex mathematical operations, such as calculating the area of a circle $A = \pi r^2$, using reusable logic.

2. Defining and Calling Functions

Definition: Functions are created using the def keyword, followed by the function name and parentheses.
Parameters: These are variables listed inside the function's parentheses, acting as placeholders for input values (arguments) that the function needs to execute its task.
Invocation (Calling): To execute a function, you simply write its name followed by parentheses, passing the required arguments.

💡 Indentation is Non-Negotiable

All code belonging to the function body must be indented (usually four spaces). Python uses this indentation, following the colon after the def statement, to determine exactly where the function block begins and ends.

# main.py - Function Definition
def add_numbers(a, b):
    # This function calculates the sum of a and b
    result = a + b
    return result
# Call the function and store the output
num1 = 10
num2 = 5
sum_result = add_numbers(num1, num2)
# Display the final result
print(f"The sum is: {sum_result}")

TERMINAL bash — 80x24

> Ready. Click "Run" to execute.

Question 1

What keyword must be used to start the definition of a function in Python?

function

define

def

setup

Question 2

The primary goal of using functions in programming is to achieve:

Strict memory allocation

Dynamic typing

Code reuse and modularity

Question 3

What component holds the input values a function needs to perform its task?

The main body

Parameters

The return statement

The colon

Challenge: Identify Duplication

Identify the opportunities for function creation.

Examine the following program logic designed to greet three different users:

*Line 1: print("Welcome, Alice!")*
*Line 2: print("Logging in...")*
*Line 3: print("Welcome, Bob!")*
*Line 4: print("Logging in...")*
*Line 5: print("Welcome, Charlie!")*
*Line 6: print("Logging in...")*

Your task is to conceptualize how a function could replace this repeating structure.

Step 1

Which lines represent highly repetitive code that should be bundled?

Solution:
The pattern Welcome, [Name]! and Logging in... is repeated for every user. Bundling this two-line sequence into a function maximizes reuse.

Step 2

What input (parameter) would this new function require?

Solution:
It would require the specific user's name (e.g., user_name), as this is the only variable changing between calls.

Step 3

Draft the theoretical function definition using only plain text.

Solution (Concept):
def greet_user(user_name):
print("Welcome, " + user_name + "!")
print("Logging in...")