Basic example

The unittest module provides a rich set of tools for constructing and running tests. This section demonstrates that a small subset of the tools suffice to meet the needs of most users.

Here is a short script to test three functions from the random module:

import random
import unittest

class TestSequenceFunctions(unittest.TestCase):

    def setUp(self):
        self.seq = range(10)

    def testshuffle(self):
        # make sure the shuffled sequence does not lose any elements
        random.shuffle(self.seq)
        self.seq.sort()
        self.assertEqual(self.seq, range(10))

    def testchoice(self):
        element = random.choice(self.seq)
        self.assert_(element in self.seq)

    def testsample(self):
        self.assertRaises(ValueError, random.sample, self.seq, 20)
        for element in random.sample(self.seq, 5):
            self.assert_(element in self.seq)

if __name__ == '__main__':
    unittest.main()

A testcase is created by subclassing unittest.TestCase. The three individual tests are defined with methods whose names start with the letters test. This naming convention informs the test runner about which methods represent tests.

The crux of each test is a call to assertEqual() to check for an expected result; assert_() to verify a condition; or assertRaises() to verify that an expected exception gets raised. These methods are used instead of the assert statement so the test runner can accumulate all test results and produce a report.

When a setUp() method is defined, the test runner will run that method prior to each test. Likewise, if a tearDown() method is defined, the test runner will invoke that method after each test. In the example, setUp() was used to create a fresh sequence for each test.

The final block shows a simple way to run the tests. unittest.main() provides a command line interface to the test script. When run from the command line, the above script produces an output that looks like this:

...
----------------------------------------------------------------------
Ran 3 tests in 0.000s

OK

Instead of unittest.main(), there are other ways to run the tests with a finer level of control, less terse output, and no requirement to be run from the command line. For example, the last two lines may be replaced with:

suite = unittest.TestLoader().loadTestsFromTestCase(TestSequenceFunctions)
unittest.TextTestRunner(verbosity=2).run(suite)

Running the revised script from the interpreter or another script produces the following output:

testchoice (__main__.TestSequenceFunctions) ... ok
testsample (__main__.TestSequenceFunctions) ... ok
testshuffle (__main__.TestSequenceFunctions) ... ok

----------------------------------------------------------------------
Ran 3 tests in 0.110s

OK

The above examples show the most commonly used unittest features which are sufficient to meet many everyday testing needs. The remainder of the documentation explores the full feature set from first principles.

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.

Re-using old test code

Some users will find that they have existing test code that they would like to run from unittest, without converting every old test function to a TestCase subclass.

For this reason, unittest provides a FunctionTestCase class. This subclass of TestCase can be used to wrap an existing test function. Set-up and tear-down functions can also be provided.

Given the following test function:

def testSomething():
    something = makeSomething()
    assert something.name is not None
    # ...

one can create an equivalent test case instance as follows:

testcase = unittest.FunctionTestCase(testSomething)

If there are additional set-up and tear-down methods that should be called as part of the test case’s operation, they can also be provided like so:

testcase = unittest.FunctionTestCase(testSomething,
                                     setUp=makeSomethingDB,
                                     tearDown=deleteSomethingDB)

To make migrating existing test suites easier, unittest supports tests raising AssertionError to indicate test failure. However, it is recommended that you use the explicit TestCase.fail*() and TestCase.assert*() methods instead, as future versions of unittest may treat AssertionError differently.

Classes and functions

class unittest.TestCase([methodName])

Instances of the TestCase class represent the smallest testable units in the unittest universe. This class is intended to be used as a base class, with specific tests being implemented by concrete subclasses. This class implements the interface needed by the test runner to allow it to drive the test, and methods that the test code can use to check for and report various kinds of failure.

Each instance of TestCase will run a single test method: the method named methodName. If you remember, we had an earlier example that went something like this:

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

Here, we create two instances of WidgetTestCase, each of which runs a single test.

methodName defaults to 'runTest'.

class unittest.FunctionTestCase(testFunc[, setUp[, tearDown[, description]]])

This class implements the portion of the TestCase interface which allows the test runner to drive the test, but does not provide the methods which test code can use to check and report errors. This is used to create test cases using legacy test code, allowing it to be integrated into a unittest-based test framework.

class unittest.TestSuite([tests])

This class represents an aggregation of individual tests cases and test suites. The class presents the interface needed by the test runner to allow it to be run as any other test case. Running a TestSuite instance is the same as iterating over the suite, running each test individually.

If tests is given, it must be an iterable of individual test cases or other test suites that will be used to build the suite initially. Additional methods are provided to add test cases and suites to the collection later on.

class unittest.TestLoader

This class is responsible for loading tests according to various criteria and returning them wrapped in a TestSuite. It can load all tests within a given module or TestCase subclass.

class unittest.TestResult

This class is used to compile information about which tests have succeeded and which have failed.

unittest.defaultTestLoader

Instance of the TestLoader class intended to be shared. If no customization of the TestLoader is needed, this instance can be used instead of repeatedly creating new instances.

class unittest.TextTestRunner([stream[, descriptions[, verbosity]]])

A basic test runner implementation which prints results on standard error. It has a few configurable parameters, but is essentially very simple. Graphical applications which run test suites should provide alternate implementations.

unittest.main([module[, defaultTest[, argv[, testRunner[, testLoader]]]]])

A command-line program that runs a set of tests; this is primarily for making test modules conveniently executable. The simplest use for this function is to include the following line at the end of a test script:

if __name__ == '__main__':
    unittest.main()

The testRunner argument can either be a test runner class or an already created instance of it.

In some cases, the existing tests may have been written using the doctest module. If so, that module provides a DocTestSuite class that can automatically build unittest.TestSuite instances from the existing doctest-based tests.

New in version 2.3.

TestCase Objects

Each TestCase instance represents a single test, but each concrete subclass may be used to define multiple tests — the concrete class represents a single test fixture. The fixture is created and cleaned up for each test case.

TestCase instances provide three groups of methods: one group used to run the test, another used by the test implementation to check conditions and report failures, and some inquiry methods allowing information about the test itself to be gathered.

Methods in the first group (running the test) are:

TestCase.setUp()

Method called to prepare the test fixture. This is called immediately before calling the test method; any exception raised by this method will be considered an error rather than a test failure. The default implementation does nothing.

TestCase.tearDown()

Method called immediately after the test method has been called and the result recorded. This is called even if the test method raised an exception, so the implementation in subclasses may need to be particularly careful about checking internal state. Any exception raised by this method will be considered an error rather than a test failure. This method will only be called if the setUp() succeeds, regardless of the outcome of the test method. The default implementation does nothing.

TestCase.run([result])

Run the test, collecting the result into the test result object passed as result. If result is omitted or None, a temporary result object is created (by calling the defaultTestCase() method) and used; this result object is not returned to run()‘s caller.

The same effect may be had by simply calling the TestCase instance.

TestCase.debug()

Run the test without collecting the result. This allows exceptions raised by the test to be propagated to the caller, and can be used to support running tests under a debugger.

The test code can use any of the following methods to check for and report failures.

TestCase.assert_(expr[, msg])

TestCase.failUnless(expr[, msg])

TestCase.assertTrue(expr[, msg])

Signal a test failure if expr is false; the explanation for the error will be msg if given, otherwise it will be None.

TestCase.assertEqual(first, second[, msg])

TestCase.failUnlessEqual(first, second[, msg])

Test that first and second are equal. If the values do not compare equal, the test will fail with the explanation given by msg, or None. Note that using failUnlessEqual() improves upon doing the comparison as the first parameter to failUnless(): the default value for msg can be computed to include representations of both first and second.

TestCase.assertNotEqual(first, second[, msg])

TestCase.failIfEqual(first, second[, msg])

Test that first and second are not equal. If the values do compare equal, the test will fail with the explanation given by msg, or None. Note that using failIfEqual() improves upon doing the comparison as the first parameter to failUnless() is that the default value for msg can be computed to include representations of both first and second.

TestCase.assertAlmostEqual(first, second[, places[, msg]])

TestCase.failUnlessAlmostEqual(first, second[, places[, msg]])

Test that first and second are approximately equal by computing the difference, rounding to the given number of decimal places (default 7), and comparing to zero. Note that comparing a given number of decimal places is not the same as comparing a given number of significant digits. If the values do not compare equal, the test will fail with the explanation given by msg, or None.

TestCase.assertNotAlmostEqual(first, second[, places[, msg]])

TestCase.failIfAlmostEqual(first, second[, places[, msg]])

Test that first and second are not approximately equal by computing the difference, rounding to the given number of decimal places (default 7), and comparing to zero. Note that comparing a given number of decimal places is not the same as comparing a given number of significant digits. If the values do not compare equal, the test will fail with the explanation given by msg, or None.

TestCase.assertRaises(exception, callable, ...)

TestCase.failUnlessRaises(exception, callable, ...)

Test that an exception is raised when callable is called with any positional or keyword arguments that are also passed to assertRaises(). The test passes if exception is raised, is an error if another exception is raised, or fails if no exception is raised. To catch any of a group of exceptions, a tuple containing the exception classes may be passed as exception.

TestCase.failIf(expr[, msg])

TestCase.assertFalse(expr[, msg])

The inverse of the failUnless() method is the failIf() method. This signals a test failure if expr is true, with msg or None for the error message.

TestCase.fail([msg])

Signals a test failure unconditionally, with msg or None for the error message.

TestCase.failureException

This class attribute gives the exception raised by the test() method. If a test framework needs to use a specialized exception, possibly to carry additional information, it must subclass this exception in order to “play fair” with the framework. The initial value of this attribute is AssertionError.

Testing frameworks can use the following methods to collect information on the test:

TestCase.countTestCases()

Return the number of tests represented by this test object. For TestCase instances, this will always be 1.

TestCase.defaultTestResult()

Return an instance of the test result class that should be used for this test case class (if no other result instance is provided to the run() method).

For TestCase instances, this will always be an instance of TestResult; subclasses of TestCase should override this as necessary.

TestCase.id()

Return a string identifying the specific test case. This is usually the full name of the test method, including the module and class name.

TestCase.shortDescription()

Returns a one-line description of the test, or None if no description has been provided. The default implementation of this method returns the first line of the test method’s docstring, if available, or None.

TestSuite Objects

TestSuite objects behave much like TestCase objects, except they do not actually implement a test. Instead, they are used to aggregate tests into groups of tests that should be run together. Some additional methods are available to add tests to TestSuite instances:

TestSuite.addTest(test)

Add a TestCase or TestSuite to the suite.

TestSuite.addTests(tests)

Add all the tests from an iterable of TestCase and TestSuite instances to this test suite.

This is equivalent to iterating over tests, calling addTest() for each element.

TestSuite shares the following methods with TestCase:

TestSuite.run(result)

Run the tests associated with this suite, collecting the result into the test result object passed as result. Note that unlike TestCase.run(), TestSuite.run() requires the result object to be passed in.

TestSuite.debug()

Run the tests associated with this suite without collecting the result. This allows exceptions raised by the test to be propagated to the caller and can be used to support running tests under a debugger.

TestSuite.countTestCases()

Return the number of tests represented by this test object, including all individual tests and sub-suites.

In the typical usage of a TestSuite object, the run() method is invoked by a TestRunner rather than by the end-user test harness.

TestResult Objects

A TestResult object stores the results of a set of tests. The TestCase and TestSuite classes ensure that results are properly recorded; test authors do not need to worry about recording the outcome of tests.

Testing frameworks built on top of unittest may want access to the TestResult object generated by running a set of tests for reporting purposes; a TestResult instance is returned by the TestRunner.run() method for this purpose.

TestResult instances have the following attributes that will be of interest when inspecting the results of running a set of tests:

TestResult.errors

A list containing 2-tuples of TestCase instances and strings holding formatted tracebacks. Each tuple represents a test which raised an unexpected exception.

Changed in version 2.2: Contains formatted tracebacks instead of sys.exc_info() results.

TestResult.failures

A list containing 2-tuples of TestCase instances and strings holding formatted tracebacks. Each tuple represents a test where a failure was explicitly signalled using the TestCase.fail*() or TestCase.assert*() methods.

Changed in version 2.2: Contains formatted tracebacks instead of sys.exc_info() results.

TestResult.testsRun

The total number of tests run so far.

TestResult.wasSuccessful()

Returns True if all tests run so far have passed, otherwise returns False.

TestResult.stop()

This method can be called to signal that the set of tests being run should be aborted by setting the TestResult‘s shouldStop attribute to True. TestRunner objects should respect this flag and return without running any additional tests.

For example, this feature is used by the TextTestRunner class to stop the test framework when the user signals an interrupt from the keyboard. Interactive tools which provide TestRunner implementations can use this in a similar manner.

The following methods of the TestResult class are used to maintain the internal data structures, and may be extended in subclasses to support additional reporting requirements. This is particularly useful in building tools which support interactive reporting while tests are being run.

TestResult.startTest(test)

Called when the test case test is about to be run.

The default implementation simply increments the instance’s testsRun counter.

TestResult.stopTest(test)

Called after the test case test has been executed, regardless of the outcome.

The default implementation does nothing.

TestResult.addError(test, err)

Called when the test case test raises an unexpected exception err is a tuple of the form returned by sys.exc_info(): (type, value, traceback).

The default implementation appends a tuple (test, formatted_err) to the instance’s errors attribute, where formatted_err is a formatted traceback derived from err.

TestResult.addFailure(test, err)

Called when the test case test signals a failure. err is a tuple of the form returned by sys.exc_info(): (type, value, traceback).

The default implementation appends a tuple (test, formatted_err) to the instance’s failures attribute, where formatted_err is a formatted traceback derived from err.

TestResult.addSuccess(test)

Called when the test case test succeeds.

The default implementation does nothing.

TestLoader Objects

The TestLoader class is used to create test suites from classes and modules. Normally, there is no need to create an instance of this class; the unittest module provides an instance that can be shared as unittest.defaultTestLoader. Using a subclass or instance, however, allows customization of some configurable properties.

TestLoader objects have the following methods:

TestLoader.loadTestsFromTestCase(testCaseClass)

Return a suite of all tests cases contained in the TestCase-derived testCaseClass.

TestLoader.loadTestsFromModule(module)

Return a suite of all tests cases contained in the given module. This method searches module for classes derived from TestCase and creates an instance of the class for each test method defined for the class.

Warning

While using a hierarchy of TestCase-derived classes can be convenient in sharing fixtures and helper functions, defining test methods on base classes that are not intended to be instantiated directly does not play well with this method. Doing so, however, can be useful when the fixtures are different and defined in subclasses.

TestLoader.loadTestsFromName(name[, module])

Return a suite of all tests cases given a string specifier.

The specifier name is a “dotted name” that may resolve either to a module, a test case class, a test method within a test case class, a TestSuite instance, or a callable object which returns a TestCase or TestSuite instance. These checks are applied in the order listed here; that is, a method on a possible test case class will be picked up as “a test method within a test case class”, rather than “a callable object”.

For example, if you have a module SampleTests containing a TestCase-derived class SampleTestCase with three test methods (test_one(), test_two(), and test_three()), the specifier 'SampleTests.SampleTestCase' would cause this method to return a suite which will run all three test methods. Using the specifier 'SampleTests.SampleTestCase.test_two' would cause it to return a test suite which will run only the test_two() test method. The specifier can refer to modules and packages which have not been imported; they will be imported as a side-effect.

The method optionally resolves name relative to the given module.

TestLoader.loadTestsFromNames(names[, module])

Similar to loadTestsFromName(), but takes a sequence of names rather than a single name. The return value is a test suite which supports all the tests defined for each name.

TestLoader.getTestCaseNames(testCaseClass)

Return a sorted sequence of method names found within testCaseClass; this should be a subclass of TestCase.

The following attributes of a TestLoader can be configured either by subclassing or assignment on an instance:

TestLoader.testMethodPrefix

String giving the prefix of method names which will be interpreted as test methods. The default value is 'test'.

This affects getTestCaseNames() and all the loadTestsFrom*() methods.

TestLoader.sortTestMethodsUsing

Function to be used to compare method names when sorting them in getTestCaseNames() and all the loadTestsFrom*() methods. The default value is the built-in cmp() function; the attribute can also be set to None to disable the sort.

TestLoader.suiteClass

Callable object that constructs a test suite from a list of tests. No methods on the resulting object are needed. The default value is the TestSuite class.

This affects all the loadTestsFrom*() methods.