Python Functions: Creating Reusable Code Blocks Like a Pro

# Calculating area of different rectangles
length1 = 5
width1 = 3
area1 = length1 * width1
print(f"Area 1: {area1}")

length2 = 8
width2 = 4
area2 = length2 * width2
print(f"Area 2: {area2}")

length3 = 10
width3 = 6
area3 = length3 * width3
print(f"Area 3: {area3}")


After Functions: The Clean Way
python
def calculate_area(length, width):
    area = length * width
    return area

# Now we can reuse our function!
print(f"Area 1: {calculate_area(5, 3)}")
print(f"Area 2: {calculate_area(8, 4)}")
print(f"Area 3: {calculate_area(10, 6)}")

def greet_user():
    """Display a simple greeting"""
    print("Hello! Welcome to Python programming!")

# Using the function
greet_user()  # Output: Hello! Welcome to Python programming!
greet_user()  # Output: Hello! Welcome to Python programming!

def make_smoothie(fruit, size):
    """Make a smoothie with specified fruit and size"""
    print(f"Making a {size} {fruit} smoothie! 🍹")
    
    if size == "large":
        print("That'll be $5.00")
    else:
        print("That'll be $3.00")

# Using our function with different parameters
make_smoothie("strawberry", "large")
make_smoothie("banana", "small")
make_smoothie("blueberry", "large")

def calculate_meal_cost(price, tip_percent=15):
    """Calculate total meal cost with tip"""
    tip_amount = price * (tip_percent / 100)
    total_cost = price + tip_amount
    return total_cost  # This sends the result back!

# Using the return value
lunch_cost = calculate_meal_cost(25)  # 15% tip default
dinner_cost = calculate_meal_cost(60, 20)  # 20% tip

print(f"Lunch: ${lunch_cost:.2f}")   # Lunch: $28.75
print(f"Dinner: ${dinner_cost:.2f}") # Dinner: $72.00

# You can use return values directly in calculations
total_spent = calculate_meal_cost(25) + calculate_meal_cost(60, 20)
print(f"Total spent: ${total_spent:.2f}")  # Total spent: $100.75

def check_password_strength(password):
    """Check if a password is strong enough"""
    if len(password) < 8:
        return "❌ Too short! Use at least 8 characters"
    elif not any(char.isdigit() for char in password):
        return "❌ Add at least one number"
    elif not any(char.isupper() for char in password):
        return "❌ Add at least one uppercase letter"
    else:
        return "✅ Strong password!"

# Test it out
print(check_password_strength("weak"))      # ❌ Too short!
print(check_password_strength("better123")) # ❌ Add uppercase
print(check_password_strength("Strong123")) # ✅ Strong password!
2. Temperature Converter
python
def celsius_to_fahrenheit(celsius):
    """Convert Celsius to Fahrenheit"""
    fahrenheit = (celsius * 9/5) + 32
    return fahrenheit

def fahrenheit_to_celsius(fahrenheit):
    """Convert Fahrenheit to Celsius"""
    celsius = (fahrenheit - 32) * 5/9
    return celsius

# Using our converters
print(f"20°C = {celsius_to_fahrenheit(20):.1f}°F")  # 20°C = 68.0°F
print(f"80°F = {fahrenheit_to_celsius(80):.1f}°C")  # 80°F = 26.7°C
3. Shopping Cart Calculator
python
def calculate_total(items, discount=0):
    """Calculate total with optional discount"""
    subtotal = sum(items)
    discount_amount = subtotal * (discount / 100)
    total = subtotal - discount_amount
    return total

# Shopping trip
prices = [15.99, 8.50, 22.75, 5.25]
regular_total = calculate_total(prices)
discounted_total = calculate_total(prices, 10)  # 10% off

print(f"Regular total: ${regular_total:.2f}")      # $52.49
print(f"With discount: ${discounted_total:.2f}")   # $47.24

# Global variables - like furniture in the living room
house_color = "blue"  # Everyone can see this

def paint_room(room_name, new_color):
    # Local variables - like items in a specific room
    old_color = house_color  # Can access global variables
    print(f"Painting {room_name} from {old_color} to {new_color}")
    
    # This only exists inside this function
    room_message = f"{room_name} is now {new_color}"
    return room_message

# Using the function
result = paint_room("kitchen", "yellow")
print(result)  # "kitchen is now yellow"

# This would cause an error - room_message doesn't exist outside the function
# print(room_message)  # ❌ NameError!
Default Parameters and Keyword Arguments
Default Parameters
python
def create_user_profile(name, age, country="Unknown", premium=False):
    """Create user profile with optional parameters"""
    profile = {
        "name": name,
        "age": age,
        "country": country,
        "premium": premium
    }
    return profile

# Using default parameters
user1 = create_user_profile("Alice", 25)  # Uses defaults for country and premium
user2 = create_user_profile("Bob", 30, "USA", True)  # Overrides defaults

print(user1)  # {'name': 'Alice', 'age': 25, 'country': 'Unknown', 'premium': False}
print(user2)  # {'name': 'Bob', 'age': 30, 'country': 'USA', 'premium': True}
Keyword Arguments
python
# You can specify parameters by name for clarity
user3 = create_user_profile(
    name="Charlie", 
    age=35, 
    premium=True, 
    country="Canada"
)

# This makes your code much more readable!
Common Function Mistakes (And How to Fix Them)
Mistake 1: Forgetting the Return Statement
python
# ❌ Wrong - function returns None
def calculate_tax(price):
    tax = price * 0.08

result = calculate_tax(100)
print(result)  # None - not what we wanted!

# ✅ Correct
def calculate_tax(price):
    tax = price * 0.08
    return tax  # Don't forget this!

result = calculate_tax(100)
print(result)  # 8.0 - much better!
Mistake 2: Modifying Mutable Parameters
python
# ❌ Dangerous - modifies the original list
def add_item(cart, item):
    cart.append(item)  # This changes the original list!
    return cart

my_cart = ["apple", "banana"]
new_cart = add_item(my_cart, "orange")
print(my_cart)  # ['apple', 'banana', 'orange'] - original changed!

# ✅ Safer - create a copy
def add_item(cart, item):
    new_cart = cart.copy()  # Work with a copy
    new_cart.append(item)
    return new_cart

my_cart = ["apple", "banana"]
new_cart = add_item(my_cart, "orange")
print(my_cart)  # ['apple', 'banana'] - original unchanged!
Your Turn: Practice Exercises
Exercise 1: Email Validator
python
def is_valid_email(email):
    """Check if an email address is valid"""
    # Your code here!
    pass

# Test cases
print(is_valid_email("user@example.com"))    # Should return True
print(is_valid_email("invalid.email"))       # Should return False
print(is_valid_email("name@domain.co.uk"))   # Should return True
Exercise 2: Number Guesser Game
python
def number_guesser(secret_number, max_attempts=5):
    """Create a number guessing game"""
    # Your code here!
    pass

# Start the game
number_guesser(42, 3)
Advanced Function Concepts (Preview)
Multiple Return Values
python
def analyze_numbers(numbers):
    """Return multiple statistics about a list of numbers"""
    minimum = min(numbers)
    maximum = max(numbers)
    average = sum(numbers) / len(numbers)
    return minimum, maximum, average  # Returns a tuple!

# Using multiple return values
scores = [85, 92, 78, 96, 88]
low, high, avg = analyze_numbers(scores)

print(f"Low: {low}, High: {high}, Average: {avg:.1f}")
Lambda Functions (Mini-functions)
python
# Quick, one-line functions
square = lambda x: x ** 2
print(square(5))  # 25

# Useful for sorting
names = ["Alice", "Bob", "Charlie", "David"]
sorted_names = sorted(names, key=lambda name: len(name))
print(sorted_names)  # ['Bob', 'Alice', 'David', 'Charlie']
We'll cover these advanced topics in future posts!

Real-World Project: Personal Task Manager
Let's build something useful with functions:

python
def create_task_manager():
    tasks = []
    
    def add_task(description, priority="medium"):
        task = {
            "id": len(tasks) + 1,
            "description": description,
            "priority": priority,
            "completed": False
        }
        tasks.append(task)
        print(f"✅ Added: {description}")
    
    def complete_task(task_id):
        for task in tasks:
            if task["id"] == task_id:
                task["completed"] = True
                print(f"🎉 Completed: {task['description']}")
                return
        print(f"❌ Task {task_id} not found")
    
    def show_tasks():
        print("\n📋 Your Tasks:")
        for task in tasks:
            status = "✓" if task["completed"] else "◯"
            print(f"{status} {task['id']}. {task['description']} [{task['priority']}]")
    
    # Return the functions we want to expose
    return add_task, complete_task, show_tasks

# Using our task manager
add, complete, show = create_task_manager()

add("Learn Python functions", "high")
add("Buy groceries", "medium")
add("Call mom", "low")

show()
complete(1)
show()