def and return

Defining a function

def area(width, height):
    return width * height

def starts the definition, then the name, then the parameters in brackets. The indented block is the body. Calling it — area(3, 4) — runs the body with width set to 3 and height set to 4.

return hands a value back and stops the function immediately. Any code after a return that runs will never execute.

A function with no return gives back None. That is the single most common source of confusion early on: print() shows a value, return gives it to the caller, and they are not the same thing. A function that only prints returns None.

Your turn: write rectangle_area(width, height) returning the area, and describe(width, height) returning a string like "3 x 4 = 12".

Returning early

Handling the awkward cases first, and returning straight away, keeps the main body of a function unindented and easy to follow:

def divide(a, b):
    if b == 0:
        return None
    return a / b

This is usually called a guard clause. Compare it with wrapping everything in an else — the guard version stays flat no matter how many cases you add.

Your turn: write safe_average(numbers) returning the mean, or None for an empty list.