7.8. Boolean Functions¶

We have already seen that boolean values result from the evaluation of boolean expressions. Since the result of any expression evaluation can be returned by a function (using the return statement), functions can return boolean values. This turns out to be a very convenient way to hide the details of complicated tests. For example:

def isDivisible(x, y):

    if x % y == 0:

        result = True

    else:

        result = False

​

    return result

​

print(isDivisible(10, 5))

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(ch06_boolfun1)

The name of this function is isDivisible. It is common to give boolean functions names that sound like yes/no questions. isDivisible returns either True or False to indicate whether the x is or is not divisible by y.

We can make the function more concise by taking advantage of the fact that the condition of the if statement is itself a boolean expression. We can return it directly, avoiding the if statement altogether:

def isDivisible(x, y):
    return x % y == 0

Boolean functions are often used in conditional statements:

if isDivisible(x, y):
    ... # do something ...
else:
    ... # do something else ...

It might be tempting to write something like if isDivisible(x, y) == True: but the extra comparison is not necessary. The following example shows the isDivisible function at work. Notice how descriptive the code is when we move the testing details into a boolean function. Try it with a few other actual parameters to see what is printed.

def isDivisible(x, y):

    if x % y == 0:

        result = True

    else:

        result = False

​

    return result

​

if isDivisible(10, 5):

    print("That works")

else:

    print("Those values are no good")

​

(ch06_boolfun2)

Here is the same program in codelens. When we evaluate the if statement in the main part of the program, the evaluation of the boolean expression causes a call to the isDivisible function. This is very easy to see in codelens.

Check your understanding

return x + y < z

​

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