What are Functions?

A Function is a reusable block of code designed to perform a specific job. Instead of copying and pasting the exact same lines of code over and over again, you package them into a function and call it whenever you need it.

Think of a function like a coffee machine:

• You give it inputs (water and coffee powder)
• It performs a job inside (brewing)
• It hands you back an output (hot coffee)

Defining and Calling a Function (def)

You define a function using the keyword def, followed by the function name, parentheses (), and a colon ::

# 1. Defining the function
def greet_user():
    print("Welcome to Python Lab!")
    print("Let's write some code.")

# 2. Calling (running) the function
greet_user()

Passing Inputs (Parameters & Arguments)

You can pass information inside parentheses so the function can use it.

• Parameter: The variable name inside the function definition waiting to receive data.
• Argument: The actual value you pass when calling the function.

# 'name' is the parameter
def greet(name):
    print(f"Hello, {name}!")

# 'Alice' is the argument
greet("Alice")
greet("Bob")

return vs. print()

A very important concept for beginners is the difference between print() and return:

• print() is only for your eyes: It shows text on the screen, but Python immediately forgets the value. You cannot save it into a variable or use it later in math.
• return hands data back to Python: It sends the computed output back to your program so you can store it inside a variable and use it downstream.

def add_numbers(a, b):
    # Return sends the result back to whoever called the function
    return a + b

# Store the returned result inside a variable
total = add_numbers(10, 15)
print(f"The total is: {total}")  # Output: The total is: 25

:::warning Return stops the function As soon as Python runs a return statement, it exits the function immediately. Any lines of code written below the return statement will never run! :::

Common Beginner Mistakes

Forgetting parentheses when calling a function

# Wrong - typing just the name without () does not run the function
greet_user

# Right - parentheses tell Python to actually execute it
greet_user()

Confusing print with return

# Wrong - print does not send data back to a variable
def add(x, y):
    print(x + y)

result = add(5, 5)
print(result)  # Outputs None!

# Right - use return to give the value back
def add(x, y):
    return x + y

What's Next?

Let's learn how to set default values for function inputs and understand how variable scope works inside and outside functions.

Default Parameters & Scope Rules

Learn optional arguments and why variables created inside functions stay private