Day 12/100: Functions in Python – def, return, and Parameters

Welcome to Day 12 of the 100 Days of Python series!
Today, we’re introducing functions — reusable blocks of code that make your programs modular, readable, and easier to debug.

By the end of this article, you'll understand how to define, call, and use parameters in functions, as well as return values using the return statement.

---

📦 What You’ll Learn

• What a function is and why it's useful
• How to define a function with def
• How to pass data using parameters
• How to get results using return
• Real-world examples of functions in action

---

🧠 What Is a Function?

A function is a block of code that only runs when you call it. It can take inputs (parameters), do some work, and optionally return a result.

Think of it as a machine: you give it something, it processes it, and gives something back.

---

🔧 1. Defining a Function with def

You define a function using the def keyword:

def greet():
    print("Hello from a function!")

To call the function, use its name followed by parentheses:

greet()

Output:

Hello from a function!

---

📨 2. Adding Parameters

Parameters let you pass data into functions.

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

Call it with an argument:

greet_user("Alice")  # Output: Hello, Alice!

You can pass multiple parameters:

def add(x, y):
    print(x + y)

add(5, 3)  # Output: 8

---

🎯 3. Returning Values with return

Use return to send a result back to the caller.

def square(number):
    return number * number

result = square(4)
print(result)  # Output: 16

You can return:

• A single value
• Multiple values as a tuple
• Even other data types like lists, dicts, etc.

---

✨ Example: Calculator Functions

def add(x, y):
    return x + y

def subtract(x, y):
    return x - y

print(add(10, 5))       # 15
print(subtract(10, 5))  # 5

---

🚀 Real-World Example 1: Tip Calculator

def calculate_tip(amount, percent):
    tip = amount * (percent / 100)
    return round(tip, 2)

print("Tip:", calculate_tip(200, 10))  # Tip: 20.0

---

🧪 Real-World Example 2: Check Even or Odd

def is_even(number):
    return number % 2 == 0

print(is_even(6))  # True
print(is_even(7))  # False

---

📌 Default Parameters

You can assign default values to parameters:

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

greet()          # Hello, stranger!
greet("Maria")   # Hello, Maria!

---

⚠️ Common Mistakes

• Forgetting to call the function with ()
• Not returning a value when you expect one
• Confusing parameters (placeholders in the function) with arguments (actual values you pass)

---

🧠 Recap

Today you learned:

• How to define a function using def
• How to use parameters to pass data
• How to use return to get a result
• Real-world examples: calculators, checkers, greeting users

---