6.4. Unit Testing¶

When we write functions that return values, we intend to use them over and over again. However, we want to be certain that they return the correct result. To be more certain these functions work correctly we write unit tests.

To write a unit test, we must know the correct result when calling the function with a specific input.

test.equal (from the test module) is a function that allows us to perform a unit test. It takes two parameters. The first is a call to the function we want to test (square in this example) with a particular input (10.0 in this example). The second parameter is the correct result that should be produced (100.0 in this example). test.equal compares what the function returns with the correct result and displays whether the unit test passes or fails.

Choosing Good Unit Tests

When we write unit tests, we should consider significantly different valid inputs to the function.

For example, the input to the square function could be either a positive or negative value. These two different kinds of inputs give us two equivalence classes of inputs. We then choose an input from each of these classes. It is important to have at least one test for each equivalence class of inputs.

Semantic errors are often caused by improperly handling the boundaries between equivalence classes. The boundary for this problem is zero. It is important to have a test at each boundary.