`test` — Regression tests package for Python¶

Note

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.support and test.regrtest. 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.

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 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__':
    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 -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):
        self.func(self.arg)

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 TestFuncAcceptsSequencesMixin 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 modules and packages whose names start with test_, importing each one, and loading its tests with unittest.TestLoader.loadTestsFromModule(). See the load_tests protocol for how a module or package can customize what is loaded. Defining a module-level test_main() function is not supported and causes the test to fail. 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.

Added in version 3.14: Output is colorized by default and can be controlled using environment variables.

`test.support` — Utilities for the Python test suite¶

The test.support module provides support for Python’s regression test suite.

Note

test.support 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:

exception 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.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.support module defines the following constants:

test.support.verbose¶: 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.

test.support.is_jython¶: True if the running interpreter is Jython.

test.support.is_android¶: True if sys.platform is android.

test.support.is_emscripten¶: True if sys.platform is emscripten.

test.support.is_wasi¶: True if sys.platform is wasi.

test.support.is_apple_mobile¶: True if sys.platform is ios, tvos, or watchos.

test.support.is_apple¶: True if sys.platform is darwin or is_apple_mobile is True.

test.support.unix_shell¶: Path for shell if not on Windows; otherwise None.

test.support.LOOPBACK_TIMEOUT¶

Timeout in seconds for tests using a network server listening on the network local loopback interface like 127.0.0.1.

The timeout is long enough to prevent test failure: it takes into account that the client and the server can run in different threads or even different processes.

The timeout should be long enough for connect(), recv() and send() methods of socket.socket.

Its default value is 10 seconds.

See also LOOPBACK_TIMEOUT.

test.support.SHORT_TIMEOUT¶

Timeout in seconds to mark a test as failed if the test takes “too long”.

The timeout value depends on the regrtest --timeout command line option.

If a test using SHORT_TIMEOUT starts to fail randomly on slow buildbots, use LONG_TIMEOUT instead.

Its default value is 30 seconds.

test.support.LONG_TIMEOUT¶

Timeout in seconds to detect when a test hangs.

It is long enough to reduce the risk of test failure on the slowest Python buildbots. It should not be used to mark a test as failed if the test takes “too long”. The timeout value depends on the regrtest --timeout command line option.

Its default value is 5 minutes.

test.support.PGO¶: Set when tests can be skipped when they are not useful for PGO.

test.support.PIPE_MAX_SIZE¶: A constant that is likely larger than the underlying OS pipe buffer size, to make writes blocking.

test.support.Py_DEBUG¶: True if Python was built with the Py_DEBUG macro defined, that is, if Python was built in debug mode.

Added in version 3.12.

test.support.SOCK_MAX_SIZE¶: A constant that is likely larger than the underlying OS socket buffer size, to make writes blocking.

test.support.TEST_SUPPORT_DIR¶: Set to the top level directory that contains test.support.

test.support.TEST_HOME_DIR¶: Set to the top level directory for the test package.

test.support.TEST_DATA_DIR¶: Set to the data directory within the test package.

test.support.MAX_Py_ssize_t¶: Set to sys.maxsize for big memory tests.

test.support.max_memuse¶: Set by set_memlimit() as the memory limit for big memory tests. Limited by MAX_Py_ssize_t.

test.support.real_max_memuse¶: Set by set_memlimit() as the memory limit for big memory tests. Not limited by MAX_Py_ssize_t.

test.support.MISSING_C_DOCSTRINGS¶

Set to True if Python is built without docstrings (the WITH_DOC_STRINGS macro is not defined). See the configure --without-doc-strings option.

See also the HAVE_DOCSTRINGS variable.

test.support.HAVE_DOCSTRINGS¶

Set to True if function docstrings are available. See the python -OO option, which strips docstrings of functions implemented in Python.

See also the MISSING_C_DOCSTRINGS variable.

test.support.TEST_HTTP_URL¶: Define the URL of a dedicated HTTP server for the network tests.

test.support.ALWAYS_EQ¶: Object that is equal to anything. Used to test mixed type comparison.

test.support.NEVER_EQ¶: Object that is not equal to anything (even to ALWAYS_EQ). Used to test mixed type comparison.

test.support.LARGEST¶: Object that is greater than anything (except itself). Used to test mixed type comparison.

test.support.SMALLEST¶: Object that is less than anything (except itself). Used to test mixed type comparison.

The test.support module defines the following functions:

test.support.busy_retry(timeout, err_msg=None, /, *, error=True)¶

Run the loop body until break stops the loop.

After timeout seconds, raise an AssertionError if error is true, or just stop the loop if error is false.

Example:

for _ in support.busy_retry(support.SHORT_TIMEOUT):
    if check():
        break

Example of error=False usage:

for _ in support.busy_retry(support.SHORT_TIMEOUT, error=False):
    if check():
        break
else:
    raise RuntimeError('my custom error')

test.support.sleeping_retry(timeout, err_msg=None, /, *, init_delay=0.010, max_delay=1.0, error=True)¶

Wait strategy that applies exponential backoff.

Run the loop body until break stops the loop. Sleep at each loop iteration, but not at the first iteration. The sleep delay is doubled at each iteration (up to max_delay seconds).

See busy_retry() documentation for the parameters usage.

Example raising an exception after SHORT_TIMEOUT seconds:

for _ in support.sleeping_retry(support.SHORT_TIMEOUT):
    if check():
        break

Example of error=False usage:

for _ in support.sleeping_retry(support.SHORT_TIMEOUT, error=False):
    if check():
        break
else:
    raise RuntimeError('my custom error')

test.support.is_resource_enabled(resource)¶: Return True if resource is enabled and available. The list of available resources is only set when test.regrtest is executing the tests.

test.support.get_resource_value(resource)¶: Return the value specified for resource (as -u resource=value). Return None if resource is disabled or no value is specified.

test.support.python_is_optimized()¶: Return True if Python was not built with -O0 or -Og.

test.support.with_pymalloc()¶: Return True if Python was built with pymalloc enabled and is not a free-threaded build.

test.support.requires(resource, msg=None)¶: 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.

test.support.sortdict(dict)¶: Return a repr of dict with keys sorted.

test.support.findfile(filename, subdir=None)¶

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.

Setting subdir indicates a relative path to use to find the file rather than looking directly in the path directories.

test.support.get_pagesize()¶: Get size of a page in bytes.

test — Regression tests package for Python¶

Writing Unit Tests for the test package¶

Running tests using the command-line interface¶

test.support — Utilities for the Python test suite¶

`test` — Regression tests package for Python¶

Writing Unit Tests for the `test` package¶

`test.support` — Utilities for the Python test suite¶