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 is_divisible(x: int, y: int) -> bool:

    result: bool

    if x % y == 0:

        result = True

    else:

        result = False

​

    return result

​

def main() -> None:

    print(str(is_divisible(10, 5)))

    return None

​

main()

​

The name of this function is is_divisible. It is common to give boolean functions names that sound like yes/no questions. is_divisible 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 is_divisible(x: int, y: int) -> bool:
    return x % y == 0

Boolean functions are often used in conditional statements:

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

It might be tempting to write something like if is_divisible(x, y) == True: but the extra comparison is redundant. You only need an == expression if you are comparing some other type than boolean. (is_divisible(x, y) == False can also be made more concise as not is_divisible(x, y)). The following example shows the is_divisible 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 is_divisible(x: int, y: int) -> bool:

    return x % y == 0

​

def main() -> None:

    if is_divisible(10, 5):

        print("That works")

    else:

        print("Those values are no good")

    return None

​

main()

​

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 is_divisible function. This is very easy to see in codelens.

Check your understanding

return x + y < z