# 6.1: Return values

- Page ID
- 40885

Calling the function generates a return value, which we usually assign to a variable or use as part of an expression.

e = math.exp(1.0) height = radius * math.sin(radians)

The functions we have written so far are void. Speaking casually, they have no return value; more precisely, their return value is `None`

.

In this chapter, we are (finally) going to write fruitful functions. The first example is `area`

, which returns the area of a circle with the given radius:

def area(radius): a = math.pi * radius**2 return a

We have seen the `return`

statement before, but in a fruitful function the return statement includes an expression. This statement means: “Return immediately from this function and use the following expression as a return value.” The expression can be arbitrarily complicated, so we could have written this function more concisely:

def area(radius): return math.pi * radius**2

On the other hand, **temporary variables** like a can make debugging easier.

Sometimes it is useful to have multiple return statements, one in each branch of a conditional:

def absolute_value(x): if x < 0: return -x else: return x

Since these return statements are in an alternative conditional, only one runs.

As soon as a return statement runs, the function terminates without executing any subsequent statements. Code that appears after a `return`

statement, or any other place the flow of execution can never reach, is called **dead code**.

In a fruitful function, it is a good idea to ensure that every possible path through the program hits a `return`

statement. For example:

def absolute_value(x): if x < 0: return -x if x > 0: return x

This function is incorrect because if `x`

happens to be 0, neither condition is `true`

, and the function ends without hitting a `return`

statement. If the flow of execution gets to the end of a function, the return value is `None`

, which is not the absolute value of 0.

>>> print(absolute_value(0)) None

By the way, Python provides a built-in function called `abs`

that computes absolute values.

As an exercise, write a compare function that takes two values, `x`

and `y`

, and returns `1`

if `x > y`

, `0`

if `x == y`

, and `-1`

if `x < y`

.