Programming Fundamentals › Programming Basics

Return Value

The result a function hands back to its caller.

Also known as: return statement, function result, return type

A return value is the result a function hands back to whoever called it.

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

size = area(3, 4)       # size is 12

return also ends the function immediately. Code after it doesn’t run.

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

Functions that return nothing

A function without return gives back a default: None in Python, undefined in JavaScript. That’s usually a bug when you meant to return something:

def add(a, b):
    a + b               # no return, so the caller gets None

Returning more than one thing

Return a tuple or an object:

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

low, high = min_max([3, 1, 4])

Habits

• Be consistent. If some paths return a value and others fall off the end, callers get surprises. Return the same kind of thing every time.
• Don’t use None as a hidden error code. Raise an exception, or document that it can return nothing (null).
• Prefer returning over modifying inputs. A function that returns its result is easier to test and reason about.
• Name functions for what they return (get_total, is_valid).
• Separate functions that do something from ones that answer something.