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
    else:
        return x

Since these return statements are in an alternative ...

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