Return Values

Why this matters

A function that only prints is a dead end: nobody can reuse its answer. A function that returns a value hands it back to the caller, who can store it, print it, or feed it into another function. This is how you build big programs from small pieces.

The idea

def double(n):
    return n * 2

answer = double(4)      # answer is 8
print(double(double(1)))  # 4

return ends the function immediately and sends a value back. Code after a return never runs.

print shows a value to a person; return gives it to the program.

def show_double(n):
    print(n * 2)       # prints, but the function's result is None

x = show_double(4)     # prints 8, then x is None

A function with no return returns None.

You can return early to handle special cases first:

def sign(n):
    if n < 0:
        return "negative"
    if n == 0:
        return "zero"
    return "positive"

And a function can return several values at once, as a tuple:

def divide(a, b):
    return a // b, a % b

quotient, remainder = divide(17, 5)   # 3 and 2

Try it

Write is_even(n) that returns True or False. Then use it in an if statement: if is_even(10):.

Your turn

Write three functions that return their answers (they must not print):

• average(numbers): the average of a list of numbers, e.g. average([2, 4, 9]) is 5.0.
• clamp(value, low, high): value, but not less than low and not more than high. clamp(15, 0, 10) is 10.
• min_max(numbers): both the smallest and the largest, as (smallest, largest).

Recap

• return value sends a result back and ends the function.
• Printing is not returning: without return the result is None.
• Return early for special cases, and return a comma-separated group to give back several values.

Common mistakes

• Printing instead of returning, so the caller gets None.
• Code after return, which never runs.
• Forgetting return on one branch, so some inputs give None.