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23.3.2 Organizing test code

The basic building blocks of unit testing are test cases -- single scenarios that must be set up and checked for correctness. In `unittest`, test cases are represented by instances of `unittest`'s `TestCase` class. To make your own test cases you must write subclasses of `TestCase`, or use `FunctionTestCase`.

An instance of a `TestCase`-derived class is an object that can completely run a single test method, together with optional set-up and tidy-up code.

The testing code of a `TestCase` instance should be entirely self contained, such that it can be run either in isolation or in arbitrary combination with any number of other test cases.

The simplest `TestCase` subclass will simply override the `runTest()` method in order to perform specific testing code:

    import unittest

    class DefaultWidgetSizeTestCase(unittest.TestCase):
        def runTest(self):
            widget = Widget('The widget')
            self.assertEqual(widget.size(), (50, 50), 'incorrect default size')

Note that in order to test something, we use the one of the `assert*()` or `fail*()` methods provided by the `TestCase` base class. If the test fails, an exception will be raised, and `unittest` will identify the test case as a failure. Any other exceptions will be treated as errors. This helps you identify where the problem is: failures are caused by incorrect results - a 5 where you expected a 6. Errors are caused by incorrect code - e.g., a `TypeError` caused by an incorrect function call.

The way to run a test case will be described later. For now, note that to construct an instance of such a test case, we call its constructor without arguments:

    testCase = DefaultWidgetSizeTestCase()

Now, such test cases can be numerous, and their set-up can be repetitive. In the above case, constructing a `Widget` in each of 100 Widget test case subclasses would mean unsightly duplication.

Luckily, we can factor out such set-up code by implementing a method called `setUp()`, which the testing framework will automatically call for us when we run the test:

    import unittest

    class SimpleWidgetTestCase(unittest.TestCase):
        def setUp(self):
            self.widget = Widget('The widget')

    class DefaultWidgetSizeTestCase(SimpleWidgetTestCase):
        def runTest(self):
            self.failUnless(self.widget.size() == (50,50),
                            'incorrect default size')

    class WidgetResizeTestCase(SimpleWidgetTestCase):
        def runTest(self):
            self.widget.resize(100,150)
            self.failUnless(self.widget.size() == (100,150),
                            'wrong size after resize')

If the `setUp()` method raises an exception while the test is running, the framework will consider the test to have suffered an error, and the `runTest()` method will not be executed.

Similarly, we can provide a `tearDown()` method that tidies up after the `runTest()` method has been run:

    import unittest

    class SimpleWidgetTestCase(unittest.TestCase):
        def setUp(self):
            self.widget = Widget('The widget')

        def tearDown(self):
            self.widget.dispose()
            self.widget = None

If `setUp()` succeeded, the `tearDown()` method will be run whether `runTest()` succeeded or not.

Such a working environment for the testing code is called a fixture.

Often, many small test cases will use the same fixture. In this case, we would end up subclassing `SimpleWidgetTestCase` into many small one-method classes such as `DefaultWidgetSizeTestCase`. This is time-consuming and discouraging, so in the same vein as JUnit, `unittest` provides a simpler mechanism:

    import unittest

    class WidgetTestCase(unittest.TestCase):
        def setUp(self):
            self.widget = Widget('The widget')

        def tearDown(self):
            self.widget.dispose()
            self.widget = None

        def testDefaultSize(self):
            self.failUnless(self.widget.size() == (50,50),
                            'incorrect default size')

        def testResize(self):
            self.widget.resize(100,150)
            self.failUnless(self.widget.size() == (100,150),
                            'wrong size after resize')

Here we have not provided a `runTest()` method, but have instead provided two different test methods. Class instances will now each run one of the `test*()` methods, with `self.widget` created and destroyed separately for each instance. When creating an instance we must specify the test method it is to run. We do this by passing the method name in the constructor:

    defaultSizeTestCase = WidgetTestCase('testDefaultSize')
    resizeTestCase = WidgetTestCase('testResize')

Test case instances are grouped together according to the features they test. `unittest` provides a mechanism for this: the test suite, represented by `unittest`'s `TestSuite` class:

    widgetTestSuite = unittest.TestSuite()
    widgetTestSuite.addTest(WidgetTestCase('testDefaultSize'))
    widgetTestSuite.addTest(WidgetTestCase('testResize'))

For the ease of running tests, as we will see later, it is a good idea to provide in each test module a callable object that returns a pre-built test suite:

    def suite():
        suite = unittest.TestSuite()
        suite.addTest(WidgetTestCase('testDefaultSize'))
        suite.addTest(WidgetTestCase('testResize'))
        return suite

or even:

    def suite():
        tests = ['testDefaultSize', 'testResize']

        return unittest.TestSuite(map(WidgetTestCase, tests))

Since it is a common pattern to create a `TestCase` subclass with many similarly named test functions, `unittest` provides a `TestLoader` class that can be used to automate the process of creating a test suite and populating it with individual tests. For example,

    suite = unittest.TestLoader().loadTestsFromTestCase(WidgetTestCase)

will create a test suite that will run `WidgetTestCase.testDefaultSize()` and `WidgetTestCase.testResize`. `TestLoader` uses the `'test'` method name prefix to identify test methods automatically.

Note that the order in which the various test cases will be run is determined by sorting the test function names with the built-in `cmp()` function.

Often it is desirable to group suites of test cases together, so as to run tests for the whole system at once. This is easy, since `TestSuite` instances can be added to a `TestSuite` just as `TestCase` instances can be added to a `TestSuite`:

    suite1 = module1.TheTestSuite()
    suite2 = module2.TheTestSuite()
    alltests = unittest.TestSuite([suite1, suite2])

You can place the definitions of test cases and test suites in the same modules as the code they are to test (such as widget.py), but there are several advantages to placing the test code in a separate module, such as test_widget.py:

- The test module can be run standalone from the command line.
- The test code can more easily be separated from shipped code.
- There is less temptation to change test code to fit the code it tests without a good reason.
- Test code should be modified much less frequently than the code it tests.
- Tested code can be refactored more easily.
- Tests for modules written in C must be in separate modules anyway, so why not be consistent?
- If the testing strategy changes, there is no need to change the source code.