25.5. test — Regression tests package for Python


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

Each module in the 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 unittest or 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.

See also

Module unittest
Writing PyUnit regression tests.
Module doctest
Tests embedded in documentation strings.

25.5.1. 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 # 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 test_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 ...

def test_main():
                              ... list other tests ...

if __name__ == '__main__':

This boilerplate code allows the testing suite to be run by test.regrtest as well as on its own as a script.

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 TestFuncAcceptsSequences(unittest.TestCase):
        func = mySuperWhammyFunction
        def test_func(self):
    class AcceptLists(TestFuncAcceptsSequences):
        arg = [1, 2, 3]
    class AcceptStrings(TestFuncAcceptsSequences):
        arg = 'abc'
    class AcceptTuples(TestFuncAcceptsSequences):
        arg = (1, 2, 3)

See also

Test Driven Development
A book by Kent Beck on writing tests before code.

25.5.2. Running tests using the command-line interface

The test.regrtest module can be run as a script to drive Python’s regression test suite, thanks to the -m option: python -m test.regrtest. 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. The names of tests to execute may also be passed to the script. Specifying a single regression test (python -m test.regrtest test_spam) will minimize output and only print whether the test passed or failed and thus minimize output.

Running test.regrtest 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.regrtest -uall,-audio,-largefile will run test.regrtest with all resources except the audio and largefile resources. For a list of all resources and more command-line options, run python -m test.regrtest -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.

25.6. test.test_support — Utility functions for tests


The test.test_support module has been renamed to test.support in Python 3.x.

The test.test_support module provides support for Python’s regression tests.

This module defines the following exceptions:

exception test.test_support.TestFailed

Exception to be raised when a test fails. This is deprecated in favor of unittest-based tests and unittest.TestCase‘s assertion methods.

exception test.test_support.ResourceDenied

Subclass of unittest.SkipTest. Raised when a resource (such as a network connection) is not available. Raised by the requires() function.

The test.test_support module defines the following constants:


True when verbose output is enabled. Should be checked when more detailed information is desired about a running test. verbose is set by test.regrtest.


True when Unicode support is available.


True if the running interpreter is Jython.


Set to a name that is safe to use as the name of a temporary file. Any temporary file that is created should be closed and unlinked (removed).

The test.test_support module defines the following functions:


Remove the module named module_name from sys.modules and delete any byte-compiled files of the module.


Return True if resource is enabled and available. The list of available resources is only set when test.regrtest is executing the tests.

test.test_support.requires(resource[, msg])

Raise ResourceDenied if resource is not available. msg is the argument to ResourceDenied if it is raised. Always returns True if called by a function whose __name__ is '__main__'. Used when tests are executed by test.regrtest.


Return the path to the file named filename. If no match is found filename is returned. This does not equal a failure since it could be the path to the file.


Execute unittest.TestCase subclasses passed to the function. The function scans the classes for methods starting with the prefix test_ and executes the tests individually.

It is also legal to pass strings as parameters; these should be keys in sys.modules. Each associated module will be scanned by unittest.TestLoader.loadTestsFromModule(). This is usually seen in the following test_main() function:

def test_main():

This will run all tests defined in the named module.

test.test_support.check_warnings(*filters, quiet=True)

A convenience wrapper for warnings.catch_warnings() that makes it easier to test that a warning was correctly raised. It is approximately equivalent to calling warnings.catch_warnings(record=True) with warnings.simplefilter() set to always and with the option to automatically validate the results that are recorded.

check_warnings accepts 2-tuples of the form ("message regexp", WarningCategory) as positional arguments. If one or more filters are provided, or if the optional keyword argument quiet is False, it checks to make sure the warnings are as expected: each specified filter must match at least one of the warnings raised by the enclosed code or the test fails, and if any warnings are raised that do not match any of the specified filters the test fails. To disable the first of these checks, set quiet to True.

If no arguments are specified, it defaults to:

check_warnings(("", Warning), quiet=True)

In this case all warnings are caught and no errors are raised.

On entry to the context manager, a WarningRecorder instance is returned. The underlying warnings list from catch_warnings() is available via the recorder object’s warnings attribute. As a convenience, the attributes of the object representing the most recent warning can also be accessed directly through the recorder object (see example below). If no warning has been raised, then any of the attributes that would otherwise be expected on an object representing a warning will return None.

The recorder object also has a reset() method, which clears the warnings list.

The context manager is designed to be used like this:

with check_warnings(("assertion is always true", SyntaxWarning),
                    ("", UserWarning)):
    exec('assert(False, "Hey!")')
    warnings.warn(UserWarning("Hide me!"))

In this case if either warning was not raised, or some other warning was raised, check_warnings() would raise an error.

When a test needs to look more deeply into the warnings, rather than just checking whether or not they occurred, code like this can be used:

with check_warnings(quiet=True) as w:
    assert str(w.args[0]) == "foo"
    assert str(w.args[0]) == "bar"
    assert str(w.warnings[0].args[0]) == "foo"
    assert str(w.warnings[1].args[0]) == "bar"
    assert len(w.warnings) == 0

Here all warnings will be caught, and the test code tests the captured warnings directly.

New in version 2.6.

Changed in version 2.7: New optional arguments filters and quiet.

test.test_support.check_py3k_warnings(*filters, quiet=False)

Similar to check_warnings(), but for Python 3 compatibility warnings. If sys.py3kwarning == 1, it checks if the warning is effectively raised. If sys.py3kwarning == 0, it checks that no warning is raised. It accepts 2-tuples of the form ("message regexp", WarningCategory) as positional arguments. When the optional keyword argument quiet is True, it does not fail if a filter catches nothing. Without arguments, it defaults to:

check_py3k_warnings(("", DeprecationWarning), quiet=False)

New in version 2.7.


This is a context manager that runs the with statement body using a StringIO.StringIO object as sys.stdout. That object can be retrieved using the as clause of the with statement.

Example use:

with captured_stdout() as s:
    print "hello"
assert s.getvalue() == "hello\n"

New in version 2.6.

test.test_support.import_module(name, deprecated=False)

This function imports and returns the named module. Unlike a normal import, this function raises unittest.SkipTest if the module cannot be imported.

Module and package deprecation messages are suppressed during this import if deprecated is True.

New in version 2.7.

test.test_support.import_fresh_module(name, fresh=(), blocked=(), deprecated=False)

This function imports and returns a fresh copy of the named Python module by removing the named module from sys.modules before doing the import. Note that unlike reload(), the original module is not affected by this operation.

fresh is an iterable of additional module names that are also removed from the sys.modules cache before doing the import.

blocked is an iterable of module names that are replaced with 0 in the module cache during the import to ensure that attempts to import them raise ImportError.

The named module and any modules named in the fresh and blocked parameters are saved before starting the import and then reinserted into sys.modules when the fresh import is complete.

Module and package deprecation messages are suppressed during this import if deprecated is True.

This function will raise unittest.SkipTest is the named module cannot be imported.

Example use:

# Get copies of the warnings module for testing without
# affecting the version being used by the rest of the test suite
# One copy uses the C implementation, the other is forced to use
# the pure Python fallback implementation
py_warnings = import_fresh_module('warnings', blocked=['_warnings'])
c_warnings = import_fresh_module('warnings', fresh=['_warnings'])

New in version 2.7.

The test.test_support module defines the following classes:

class test.test_support.TransientResource(exc[, **kwargs])

Instances are a context manager that raises 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 with statement. Only if all pairs match properly against attributes on the exception is ResourceDenied raised.

New in version 2.6.

class test.test_support.EnvironmentVarGuard

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.

New in version 2.6.

Changed in version 2.7: Added dictionary interface.

EnvironmentVarGuard.set(envvar, value)

Temporarily set the environment variable envvar to the value of value.


Temporarily unset the environment variable envvar.

class test.test_support.WarningsRecorder

Class used to record warnings for unit tests. See documentation of check_warnings() above for more details.

New in version 2.6.