Imagine writing a piece of code once and using it over and over again without copying and pasting.

That’s the power of functions.

In this lesson, you’ll learn:

• What functions are and why they matter
• How to write custom functions
• How to pass data (arguments) into functions
• How to get data out (return values)
• The difference between built-in vs user-defined functions
• How to write clean, reusable, readable code using functions

Let’s go step by step, with tons of examples.

🧠 What Is a Function?

A function is a reusable block of code that does something.

You define it once, and then call it whenever you need it.

Think of it like a blender:

• You give it ingredients (inputs)
• It processes them
• And returns a smoothie (output)

✅ How to Define a Function

def greet():
    print("Hello! Welcome to PyUniverse.")

• def starts the function
• greet is the function name
• () holds parameters (more on that later)
• Indented block is what the function does

📞 Calling a Function

Once defined, you call the function by its name:

greet()

Output:

Hello! Welcome to PyUniverse.

📥 Passing Data to Functions (Parameters)

def greet_user(name):
    print(f"Hello, {name}! Welcome to PyUniverse.")

You can pass different names:

greet_user("Ali")
greet_user("Fatima")

Output:

Hello, Ali! Welcome to PyUniverse.
Hello, Fatima! Welcome to PyUniverse.

🔢 Multiple Parameters

def add(a, b):
    print("Sum:", a + b)

add(5, 3)
add(100, 20)

Output:

Sum: 8
Sum: 120

📤 Returning Data from Functions

Functions don’t have to just print they can return a result.

def multiply(x, y):
    return x * y

result = multiply(4, 6)
print("Product is:", result)

Output:

Product is: 24

return sends the result back to wherever the function was called.

🧩 Example: BMI Calculator

def calculate_bmi(weight, height):
    bmi = weight / (height ** 2)
    return round(bmi, 2)

my_bmi = calculate_bmi(68, 1.75)
print("Your BMI is:", my_bmi)

🚫 Return vs Print

def say_hello():
    return "Hello"

x = say_hello()
print(x)

return gives you a value you can reuse.
print() just displays something on screen it doesn’t store anything.

🧪 Real Use: Age Check

def is_adult(age):
    return age >= 18

print(is_adult(21))  # True
print(is_adult(16))  # False

🧠 Default Arguments

You can assign default values to parameters:

def greet(name="Guest"):
    print(f"Hi, {name}!")

greet("Sara")
greet()

Output:

Hi, Sara!
Hi, Guest!

💡 Keyword Arguments

Order doesn’t matter when you use names:

def profile(name, age):
    print(f"{name} is {age} years old.")

profile(age=25, name="Aamir")

⚙️ Functions That Call Other Functions

def square(x):
    return x * x

def print_square(x):
    result = square(x)
    print(f"The square of {x} is {result}")

print_square(7)

🔁 Reusability in Action

def print_line():
    print("-" * 30)

print("Shopping List")
print_line()
print("Milk\nEggs\nBread")
print_line()

🔍 Built-in vs User-Defined Functions

⚠️ Common Mistakes