18.1. Introduction: Test Cases

A test case expresses requirements for a program, in a way that can be checked automatically. Specifically, a test asserts something about the state of the program at a particular point in its execution.

We have previously suggested that it’s a good idea to first write down comments about what your code is supposed to do, before actually writing the code. It is an even better idea to write down some test cases before writing a program. For example, before writing a function, write a few test cases that check that it returns an object of the right type and that it returns the correct values when invoked on particular inputs.

There are several reasons why it’s a good habit to write test cases.

Now it’s time to learn how to write code for test cases, or unit tests. To write one, we must know what we expect some value to be at a particular point in the program’s execution. For example, we need to specify what the correct result would be when calling the function with a specific input.

testEqual (from the test module) is a function that allows us to perform a unit test. It takes two parameters. In the example above, the first is a call to the function we want to test (square in this example) with a particular input (10 in this example). The second is the correct result that should be produced (100 in this example). test.testEqual compares the two values and displays a message about whether the unit test passes or fails: pass if the two values are equal, fail if not.

Extend the program …

On line 8, write another unit test (that should pass) for the square function.


The test module is not a standard Python module. Instead, there are other more powerful and more modern modules, such as one called unittest which will be taught in more advanced courses. However, the test module offers a simple introduction to testing that is appropriate at this stage in the interactive text.

Check your understanding

    test-1-1: When test.testEqual() is passed two values that are not the same, it generates an error and stops execution of the program.

  • True
  • A message is printed out, but the program does not stop executing
  • False
  • A message is printed out, but the program does not stop executing
  • It depends
  • A message is printed out, but the program does not stop executing

    test-1-2: Test cases are a waste of time, because the python interpreter will give an error message when the program runs incorrectly, and that’s all you need for debugging.

  • True
  • You might not notice the error, if the code just produces a wrong output rather generating an error. And it may be difficult to figure out the original cause of an error when you do get one.
  • False
  • Test cases let you test some pieces of code as you write them, rather than waiting for problems to show themselves later.

    test-1-3: For the hangman game blanked function, which of the following is the correct way to write a test to check that ‘under’ will be blanked as 'u_d__' when the user has guessed letters d and u so far?

    def blanked(word, revealed_letters):
        return word
    import test
    test.testEqual(blanked('hello', 'elj'), "_ell_")
    test.testEqual(blanked('hello', ''), '_____')
    test.testEqual(blanked('ground', 'rn'), '_r__n_')
    test.testEqual(blanked('almost', 'vrnalmqpost'), 'almost')
  • test.testEqual(blanked('under', 'du', 'u_d__'))
  • blanked only takes two inputs; this provides three inputs to the blanked function
  • test.testEqual(blanked('under', 'u_d__'), 'du')
  • The second argument to the blanked function should be the letters that have been guessed, not the blanked version of the word
  • test.testEqual(blanked('under', 'du'), 'u_d__')
  • This checks whether the value returned from the blanked function is 'u_d__'.
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