# 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:

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 redundant. You only need an `==` expression if you are comparing some other type than boolean. (`isDivisible(x, y) == False` can also be made more concise as `not isDivisible(x, y)`). 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.

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.

Activity: CodeLens 7.8.3 (ch06_boolcodelens)

## 7.8.1. More Unit Testing¶

When we write unit tests, we should also consider output equivalence classes that result in significantly different results.

The `isDivisible` function can return either `True` or `False`. These two different outputs give us two equivalence classes. We then choose inputs that should give each of the different results. It is important to have at least one test for each output equivalence class.

Extend the program …

Starting on line 7, write two unit tests (that should pass), one for each output equivalence class.

Note

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