6.1: Return values

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.