6.1: Return values
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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
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,
y, and returns
x > y,
x == y, and
x < y.