Writing tests

For any module ‘somepkg.somemod’ there should be a corresponding unit test module ‘somepkg.somemod.tests.test_somemod’. Or if more than one set of unit tests is desired, multiple test modules of the form ‘somepkg.somemod.tests.test_somemodYYYY’. Note that this means that your ‘somemod’ directory needs to have a ‘tests’ subdirectory, and that that subdirectory must have a (normally empty) ‘__init__.py’ file in it.

In your unit test class, begin all unit test methods with the string ‘test’. If you use the CleanUp class support, make sure that your ‘setUp’ and ‘tearDown’ methods call the CleanUp class’s ‘setUp’ and ‘tearDown’ methods:

class TestSomething(CleanUp):
    def setUp(self):
        #your setup here

    def tearDown(self):
        #your teardown here

Never give your test methods a docstring! Doing so makes it very difficult to find your test method when using the verbose output. Use a comment instead.

Call your test class TestSomething, never just Test.

Call your test methods test_something(), not testSomething()

The typical approach to structuring the tests themselves is to write test methods that exercise each individual method of the class. It is a good idea to organize these tests according to the Interfaces implemented by the class under test. In fact, it is often best to implement such tests in separate mixin classes, one class per Interface.

Within the unit tests themselves, the Zope style is to use the positive rather than the double negative assertions. Thus, use assertEqual rather than failUnlessEqual, assertRaises rather than failUnlessRaises, and assert_ rather than failUnless. (Yes, assert_ is an ugly name, but it is still preferred.)

There are certain other “Best Practices” the following of which leads to more robust and general unit tests:

  • If at all possible, avoid writing to the file system. It should be possible to run the tests with only read-only access to the test directories. If you do have to create files, create them using the python tempfile module. Be sure to clean up after yourselves in your tearDown method.
  • It has been suggested that doing import of the module under test in the global section of the test module is bad, because in some uses of unittest test modules that generate import errors outside the tests themselves are ignored silently. It has also been suggested that if so this is a bug in unittest. I’m not aware of a definitive answer to this question. If you wish to avoid globally importing the module under test, you can write a “helper function” to do the import and create and return instance(s) of the objects needed for testing, and call it from the start of each unit test.