Parameters and return

Big idea

return sends a value back to whoever called the function. Parameters can have default values, and arguments can be passed by name.

return

def square(n):
    return n * n

result = square(5)
print(result)
print(square(3) + square(4))

output
25
25

The call square(5) is replaced by the value it returns, so you can store it, print it, or use it in more maths.

return also ends the function immediately — anything after it in that path never runs.

print is not return

This is the most common confusion with functions:

def shout(word):
    print(word.upper())

result = shout("hi")
print(result)

HI
None

shout prints, but it doesn't return anything — and a function with no return gives back None. Return a value when the caller needs to use it; print only when the job is to show something.

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

total = add(10, 20)
print("Total:", total)

output
Total: 30

Several parameters, and defaults

def power(base, exponent=2):
    return base ** exponent

print(power(5))
print(power(2, 10))

output
25
1024

exponent=2 is a default: used when the caller doesn't pass that argument. Parameters with defaults must come after those without.

Keyword arguments

You can pass arguments by name. Then order doesn't matter, and the call explains itself:

def describe(name, age, city):
    return f"{name}, {age}, from {city}"

describe("Ada", 36, "London")                  # by position
describe(city="Paris", name="Marie", age=66)   # by name

Returning several values

Return a tuple and unpack it on the other side:

def min_max(numbers):
    return min(numbers), max(numbers)

low, high = min_max([7, 2, 9, 4])
print(low, high)

output
2 9

Watch out: a list as a default

Default values are made once, when the def runs. A list default is shared between calls — surprising:

def add_item(item, items=[]):      # ✗ the same list every call
    items.append(item)
    return items

def add_item(item, items=None):    # ✓ a fresh list each time
    if items is None:
        items = []
    items.append(item)
    return items

Watch it

Think of it like

A vending machine. You put in arguments (coins and a button), and it returns a snack into the tray for you to take away. A machine that only flashes "Enjoy!" on its screen (print) leaves your hands empty.

Key terms

• return — Sends a value back to the caller and ends the function.
• Default parameter — A parameter with a value used when no argument is given.
• Keyword argument — An argument passed by name: power(base=2).