The Python unit testing framework, sometimes referred to as “PyUnit,” is a Python language version of JUnit, by Kent Beck and Erich Gamma. JUnit is, in turn, a Java version of Kent’s Smalltalk testing framework. Each is the de facto standard unit testing framework for its respective language.
unittest supports test automation, sharing of setup and shutdown code for tests, aggregation of tests into collections, and independence of the tests from the reporting framework. The unittest module provides classes that make it easy to support these qualities for a set of tests.
To achieve this, unittest supports some important concepts:
The test case and test fixture concepts are supported through the TestCase and FunctionTestCase classes; the former should be used when creating new tests, and the latter can be used when integrating existing test code with a unittest-driven framework. When building test fixtures using TestCase, the setUp() and tearDown() methods can be overridden to provide initialization and cleanup for the fixture. With FunctionTestCase, existing functions can be passed to the constructor for these purposes. When the test is run, the fixture initialization is run first; if it succeeds, the cleanup method is run after the test has been executed, regardless of the outcome of the test. Each instance of the TestCase will only be used to run a single test method, so a new fixture is created for each test.
Test suites are implemented by the TestSuite class. This class allows individual tests and test suites to be aggregated; when the suite is executed, all tests added directly to the suite and in “child” test suites are run.
A test runner is an object that provides a single method, run(), which accepts a TestCase or TestSuite object as a parameter, and returns a result object. The class TestResult is provided for use as the result object. unittest provides the TextTestRunner as an example test runner which reports test results on the standard error stream by default. Alternate runners can be implemented for other environments (such as graphical environments) without any need to derive from a specific class.
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.
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
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:
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.
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.
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'.
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.
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.
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:
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.
The test code can use any of the following methods to check for and report failures.
Testing frameworks can use the following methods to collect information on the test:
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).
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:
This is equivalent to iterating over tests, calling addTest() for each element.
In the typical usage of a TestSuite object, the run() method is invoked by a TestRunner rather than by the end-user test harness.
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:
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.
Called when the test case test is about to be run.
The default implementation simply increments the instance’s testsRun counter.
Called after the test case test has been executed, regardless of the outcome.
The default implementation does nothing.
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.
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.
Called when the test case test succeeds.
The default implementation does nothing.
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:
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.
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.
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.
The following attributes of a TestLoader can be configured either by subclassing or assignment on an instance:
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.