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# 25.5. `test` — Regression tests package for Python

Warning

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.

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 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():
        support.run_unittest(MyTestCase1,
                                  MyTestCase2,
                                  ... list other tests ...
                                 )

    if __name__ == '__main__':
        test_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):
              self.func(self.arg)

      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. Run python -m test.regrtest -uall to turn on all resources; specifying `all` as an option for `-u` enables all possible resources. 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.support` — Utility functions for tests

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

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.``TestSkipped`
  Subclass of `TestFailed`. Raised when a test is skipped. This occurs when a needed resource (such as a network connection) is not available at the time of testing.

exception `test.support.``ResourceDenied`
  Subclass of `TestSkipped`. 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.``TESTFN`
  Set to the path that a temporary file may be created at. Any temporary that is created should be closed and unlinked (removed).

The `test.support` module defines the following functions:

`test.support.``forget`(module_name)
  Removes the module named module_name from `sys.modules` and deletes any byte-compiled files of the module.

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

`test.support.``requires`(resource, msg=None)
  Raises `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.``findfile`(filename)
  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.

`test.support.``run_unittest`(*classes)

  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():
          support.run_unittest(__name__)

  This will run all tests defined in the named module.

`test.support.``check_warnings`()

  A convenience wrapper for `warnings.catch_warnings()` that makes it easier to test that a warning was correctly raised with a single assertion. It is approximately equivalent to calling `warnings.catch_warnings(record=True)`.

  The main difference is that on entry to the context manager, a `WarningRecorder` instance is returned instead of a simple list. The underlying warnings list is available via the recorder object’s `warnings` attribute, while the attributes of the last raised warning are also accessible directly on the object. If no warning has been raised, then the latter attributes will all be `None`.

  A `reset()` method is also provided on the recorder object. This method simply clears the warning list.

  The context manager is used like this:

      with check_warnings() as w:
          warnings.simplefilter("always")
          warnings.warn("foo")
          assert str(w.message) == "foo"
          warnings.warn("bar")
          assert str(w.message) == "bar"
          assert str(w.warnings[0].message) == "foo"
          assert str(w.warnings[1].message) == "bar"
          w.reset()
          assert len(w.warnings) == 0

`test.support.``captured_stdout`()

  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"

`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 3.1.

`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 3.1.

The `test.support` module defines the following classes:

class `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.

class `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.

  Changed in version 3.1: Added dictionary interface.

`EnvironmentVarGuard.``set`(envvar, value)
  Temporarily set the environment variable `envvar` to the value of `value`.

`EnvironmentVarGuard.``unset`(envvar)
  Temporarily unset the environment variable `envvar`.

class `test.support.``WarningsRecorder`
  Class used to record warnings for unit tests. See documentation of `check_warnings()` above for more details.

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