Writing Unit Tests for the test package¶

It is preferred that tests that use the unittest module follow a few

guidelines. One is to name the test module by starting it with test_ and end

it with the name of the module being tested. The test methods in the test module

should start with test_ and end with a description of what the method is

testing. This is needed so that the methods are recognized by the test driver as

test methods. Also, no documentation string for the method should be included. A

comment (such as #TestsfunctionreturnsonlyTrueorFalse) should be used

to provide documentation for test methods. This is done because documentation

strings get printed out if they exist and thus what test is being run is not

stated.

A basic boilerplate is often used:

importunittest
fromtestimportsupport
classMyTestCase1(unittest.TestCase):
# Only use setUp() and tearDown() if necessary
defsetUp(self):
...codetoexecuteinpreparationfortests...
deftearDown(self):
...codetoexecutetocleanupaftertests...
deftest_feature_one(self):
# Test feature one.
...testingcode...
deftest_feature_two(self):
# Test feature two.
...testingcode...
...moretestmethods...
classMyTestCase2(unittest.TestCase):
...samestructureasMyTestCase1...
...moretestclasses...
if__name__=='__main__':
unittest.main()

This code pattern allows the testing suite to be run by test.regrtest,

on its own as a script that supports the unittest CLI, or via the

python-munittest CLI.

The goal for regression testing is to try to break code. This leads to a few

guidelines to be followed:

• The testing suite should exercise all classes, functions, and constants. This

  includes not just the external API that is to be presented to the outside

  world but also "private" code.

• Whitebox testing (examining the code being tested when the tests are being

  written) is preferred. Blackbox testing (testing only the published user

  interface) is not complete enough to make sure all boundary and edge cases

  are tested.

• Make sure all possible values are tested including invalid ones. This makes

  sure that not only all valid values are acceptable but also that improper

  values are handled correctly.

• Exhaust as many code paths as possible. Test where branching occurs and thus

  tailor input to make sure as many different paths through the code are taken.

• Add an explicit test for any bugs discovered for the tested code. This will

  make sure that the error does not crop up again if the code is changed in the

  future.

• Make sure to clean up after your tests (such as close and remove all temporary

  files).

• If a test is dependent on a specific condition of the operating system then

  verify the condition already exists before attempting the test.

• Import as few modules as possible and do it as soon as possible. This

  minimizes external dependencies of tests and also minimizes possible anomalous

  behavior from side-effects of importing a module.

• Try to maximize code reuse. On occasion, tests will vary by something as small

  as what type of input is used. Minimize code duplication by subclassing a

  basic test class with a class that specifies the input:

  python3 notranslate">

  classTestFuncAcceptsSequencesMixin:
  func=mySuperWhammyFunction
  deftest_func(self):
  self.func(self.arg)
  classAcceptLists(TestFuncAcceptsSequencesMixin,unittest.TestCase):
  arg=[1,2,3]
  classAcceptStrings(TestFuncAcceptsSequencesMixin,unittest.TestCase):
  arg='abc'
  classAcceptTuples(TestFuncAcceptsSequencesMixin,unittest.TestCase):
  arg=(1,2,3)
  

  When using this pattern, remember that all classes that inherit from

  unittest.TestCase are run as tests. The Mixin class in the example above

  does not have any data and so can't be run by itself, thus it does not

  inherit from unittest.TestCase.

Running tests using the command-line interface¶

The test package can be run as a script to drive Python's regression

test suite, thanks to the -m option: python -m test. Under

the hood, it uses test.regrtest; the call python -m

test.regrtest used in previous Python versions still works. Running the

script by itself automatically starts running all regression tests in the

test package. It does this by finding all modules in the package whose

name starts with test_, importing them, and executing the function

test_main() if present or loading the tests via

unittest.TestLoader.loadTestsFromModule if test_main does not exist. The

names of tests to execute may also be passed to the script. Specifying a single

regression test (python -m test test_spam) will minimize output and

only print whether the test passed or failed.

Running test directly allows what resources are available for

tests to use to be set. You do this by using the -u command-line

option. Specifying all as the value for the -u option enables all

possible resources: python -m test -uall.

If all but one resource is desired (a more common case), a

comma-separated list of resources that are not desired may be listed after

all. The command python -m test -uall,-audio,-largefile

will run test with all resources except the audio and

largefile resources. For a list of all resources and more command-line

options, run python -m test -h.

Some other ways to execute the regression tests depend on what platform the

tests are being executed on. On Unix, you can run make test at the

top-level directory where Python was built. On Windows,

executing rt.bat from your PCbuild directory will run all

regression tests.