Lesson 26: User Defined Functions

Introduction to Functions

Functions are reusable blocks of code that perform specific tasks. While Python has many built-in functions like print() and len(), creating your own functions allows you to organize code, avoid repetition, and make programs more modular and easier to maintain.

Functions help you write cleaner, more organized code by grouping related operations together and allowing you to call them whenever needed.

Defining Your First Function

To create a function, use the def keyword followed by the function name, parentheses, and a colon. The function body is indented below the definition.

basic_function.py
# Define a simple function
def greet():
    print("Hello, World!")

# Call the function
greet()  # Output: Hello, World!
greet()  # Output: Hello, World!

💡 Key Points

• Function definition uses def keyword

• Function name follows Python naming conventions (lowercase with underscores)

• Empty parentheses () mean the function takes no parameters

• Function code is indented

Functions with Parameters

Parameters allow functions to accept input values. When you define a function, you specify parameters inside the parentheses. When you call the function, you provide arguments (the actual values).

parameters.py
# Function with one parameter
def greet(name):
    print(f"Hello, {name}!")

greet("Alice")  # Output: Hello, Alice!
greet("Bob")    # Output: Hello, Bob!

# Function with multiple parameters
def introduce(name, age):
    print(f"I'm {name}, and I'm {age} years old.")

introduce("Charlie", 25)  # Output: I'm Charlie, and I'm 25 years old.

Why Use Functions?

Functions provide several important benefits:

Code Reusability - Write once, use many times
Organization - Group related code together
Maintainability - Fix bugs in one place
Readability - Descriptive function names explain purpose
Modularity - Break complex problems into smaller parts

Function Examples

Here are practical examples of useful functions:

examples.py
# Calculate area of a rectangle
def calculate_area(width, height):
    area = width * height
    print(f"Area: {area}")

calculate_area(5, 3)  # Output: Area: 15

# Check if a number is even
def is_even(number):
    if number % 2 == 0:
        print("Even")
    else:
        print("Odd")

is_even(4)  # Output: Even
is_even(7)  # Output: Odd

# Display a formatted message
def display_info(name, score):
    print(f"Player: {name}")
    print(f"Score: {score}")
    print("-" * 20)

display_info("Alice", 95)

Best Practices

✅ Function Naming Tips

• Use descriptive names that explain what the function does

• Use lowercase letters and underscores (snake_case)

• Start with a verb: calculate, display, check

• Keep names concise but clear

💡 Important Notes

• Functions must be defined before they're called

• Parameters are local variables that only exist inside the function

• You can call a function as many times as needed

User Defined Functions