Functions

Write Once, Use Anywhere – The Power of Functions

As your programs get larger, repeating code becomes messy. That’s where functions come in.

Functions help you:

• Reuse code easily
• Break big problems into smaller pieces
• Write cleaner, maintainable programs

1. 🧪 Defining and Calling Functions #

A function is defined using the def keyword.

def greet():
    print("Hello, Hannan!")

Calling the function:

greet()

💡 Why use functions?
Imagine needing to greet users in 10 places — just call greet() instead of copy-pasting the print line.

2. 🎁 Parameters, Arguments, and Default Values #

🧾 Parameters #

These are placeholders in the function definition.

📨 Arguments #

These are actual values you pass when calling the function.

def greet(name):
    print("Hello,", name)

greet("Hannan")  # Output: Hello, Hannan

⚙️ Default Parameters #

Provide a default value that gets used if no argument is passed.

def greet(name="Guest"):
    print("Hello,", name)

greet()            # Output: Hello, Guest
greet("Hannan")    # Output: Hello, Hannan

3. 🧾 Return Statements #

Functions can return values instead of just printing them.

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

result = add(5, 3)
print("Sum:", result)  # Output: Sum: 8

Without return, functions just perform an action — but with return, they give back results you can use later.

4. 📦 Variable Scope: Local vs. Global #

Understanding where your variables “live” is key to writing bug-free code.

🔒 Local Scope #

Variables declared inside functions.

def test():
    x = 10  # local variable
    print(x)

test()
# print(x)  # ❌ Error: x is not defined outside

🌐 Global Scope #

Variables declared outside functions are available globally.

x = 5

def show():
    print(x)  # accessing global variable

show()  # Output: 5

✏️ Modifying Global Variables #

You must use global keyword to change a global variable inside a function.

x = 10

def modify():
    global x
    x = 20

modify()
print(x)  # Output: 20

💼 Real-World Use Case: Bill Calculator Using Functions #

Let’s create a Bill Calculator for a store or restaurant using functions.

🔧 Features: #

• Accepts multiple items with price and quantity
• Calculates total
• Adds tax
• Prints final amount

💻 Full Code: #

# Function to calculate total bill for an item
def calculate_item_total(price, quantity):
    return price * quantity

# Function to apply tax
def apply_tax(total, tax_rate=0.10):  # 10% tax by default
    return total + (total * tax_rate)

# Main function to run the bill calculator
def bill_calculator():
    total = 0
    print("🧾 Welcome to Hannan's Store")
    
    while True:
        item = input("Enter item name (or type 'done' to finish): ")
        if item.lower() == 'done':
            break
        try:
            price = float(input("Enter price of the item: "))
            quantity = int(input("Enter quantity: "))
            item_total = calculate_item_total(price, quantity)
            total += item_total
            print(f"{item.title()} total: Rs.{item_total:.2f}\n")
        except ValueError:
            print("❌ Invalid input. Please enter numbers only.\n")

    total_with_tax = apply_tax(total)
    print(f"\n💰 Final Total (with tax): Rs.{total_with_tax:.2f}")

# Run the calculator
bill_calculator()

🧪 Output Sample: #

🧾 Welcome to Hannan's Store
Enter item name (or type 'done' to finish): Apple
Enter price of the item: 50
Enter quantity: 3
Apple total: Rs.150.00

Enter item name (or type 'done' to finish): done

💰 Final Total (with tax): Rs.165.00

🧠 What You Learned: #

💡 Bonus Tips: #

• Use functions for:
  • Validations
  • Clean UI formatting
  • Repeated calculations
• Group related functions in modules
• Use *args and **kwargs for more dynamic functions (coming in advanced topics!)

🚀 What’s Next? #

Coming up next:

🔄 Lists & Collection Handling
Learn how to use Python’s most powerful built-in collections — with a real-world use case like Student Marks Manager

Want me to continue with that now? Or package this chapter for sharing or training material? Just say the word!