What Is a Function?

A function is a reusable block of code that performs a specific task. You write it once, and then call it whenever you need it.

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

You call it like this:

greet()
      

Functions with Parameters

Parameters allow you to send data into a function.

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

greet("Kaloyan")
      

Functions That Return Values

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

result = add(5, 3)
print(result)   # 8
      

return sends a value back to wherever the function was called.

Default Values

def welcome(name="Guest"):
    print("Welcome,", name)

welcome()
welcome("Kaloyan")
      

What Is a Procedure?

A procedure is just a function that does not return anything.

def log(message):
    print("[LOG]", message)
      

Why Functions Matter

• Cleaner code
• No repetition
• Easier debugging
• Reusable components

Example — Calculator

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

def subtract(a, b):
    return a - b

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

What’s Next?

Next you’ll learn how to build simple webpages using HTML.