python-clinic / Doc / library / test.rst

:mod:`test` --- Regression tests package for Python


The :mod:`test` package is meant for internal use by Python only. It is documented for the benefit of the core developers of Python. Any use of this package outside of Python's standard library is discouraged as code mentioned here can change or be removed without notice between releases of Python.

The :mod:`test` package contains all regression tests for Python as well as the modules :mod:`` and :mod:`test.regrtest`. :mod:`` is used to enhance your tests while :mod:`test.regrtest` drives the testing suite.

Each module in the :mod:`test` package whose name starts with test_ is a testing suite for a specific module or feature. All new tests should be written using the :mod:`unittest` or :mod:`doctest` module. Some older tests are written using a "traditional" testing style that compares output printed to sys.stdout; this style of test is considered deprecated.

Writing Unit Tests for the :mod:`test` package

It is preferred that tests that use the :mod:`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 # Tests function returns only True or False) 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:

import unittest
from test import support

class MyTestCase1(unittest.TestCase):

    # Only use setUp() and tearDown() if necessary

    def setUp(self):
        ... code to execute in preparation for tests ...

    def tearDown(self):
        ... code to execute to clean up after tests ...

    def test_feature_one(self):
        # Test feature one.
        ... testing code ...

    def test_feature_two(self):
        # Test feature two.
        ... testing code ...

    ... more test methods ...

class MyTestCase2(unittest.TestCase):
    ... same structure as MyTestCase1 ...

... more test classes ...

if __name__ == '__main__':

This code pattern allows the testing suite to be run by :mod:`test.regrtest`, on its own as a script that supports the :mod:`unittest` CLI, or via the python -m unittest 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:

    class TestFuncAcceptsSequencesMixin:
        func = mySuperWhammyFunction
        def test_func(self):
    class AcceptLists(TestFuncAcceptsSequencesMixin, unittest.TestCase):
        arg = [1, 2, 3]
    class AcceptStrings(TestFuncAcceptsSequencesMixin, unittest.TestCase):
        arg = 'abc'
    class AcceptTuples(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 :mod:`test` package can be run as a script to drive Python's regression test suite, thanks to the :option:`-m` option: :program:`python -m test`. Under the hood, it uses :mod:`test.regrtest`; the call :program:`python -m test.regrtest` used in previous Python versions still works). Running the script by itself automatically starts running all regression tests in the :mod:`test` package. It does this by finding all modules in the package whose name starts with test_, importing them, and executing the function :func:`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 (:program:`python -m test test_spam`) will minimize output and only print whether the test passed or failed.

Running :mod:`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: :program:`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 :program:`python -m test -uall,-audio,-largefile` will run :mod:`test` with all resources except the audio and largefile resources. For a list of all resources and more command-line options, run :program:`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 :program:`make test` at the top-level directory where Python was built. On Windows, executing :program:`rt.bat` from your :file:`PCBuild` directory will run all regression tests.

:mod:`` --- Utilities for the Python test suite

The :mod:`` module provides support for Python's regression test suite.


:mod:`` is not a public module. It is documented here to help Python developers write tests. The API of this module is subject to change without backwards compatibility concerns between releases.

This module defines the following exceptions:

The :mod:`` module defines the following constants:

The :mod:`` module defines the following functions:

The :mod:`` module defines the following classes:

Instances are a context manager that raises :exc:`ResourceDenied` if the specified exception type is raised. Any keyword arguments are treated as attribute/value pairs to be compared against any exception raised within the :keyword:`with` statement. Only if all pairs match properly against attributes on the exception is :exc:`ResourceDenied` raised.

Class used to temporarily set or unset environment variables. Instances can be used as a context manager and have a complete dictionary interface for querying/modifying the underlying os.environ. After exit from the context manager all changes to environment variables done through this instance will be rolled back.

Class used to record warnings for unit tests. See documentation of :func:`check_warnings` above for more details.