Chapter 6. Fruitful Functions

Many of the Python functions we have used, such as the math functions, produce return values. But the functions we’ve written are all void: they have an effect, like printing a value or moving a turtle, but they don’t have a return value. In this chapter you will learn to write fruitful functions.

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
        return x

Since these return statements are in an alternative ...

Get Think Python, 2nd Edition now with the O’Reilly learning platform.

O’Reilly members experience live online training, plus books, videos, and digital content from nearly 200 publishers.