cpython/Doc/library/test.rst

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

.. module:: test
   :synopsis: Regression tests package containing the testing suite for Python.
.. sectionauthor:: Brett Cannon <brett@python.org>

.. note::
   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:`test.support` and :mod:`test.regrtest`.
:mod:`test.support` 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.


.. seealso::

   Module :mod:`unittest`
      Writing PyUnit regression tests.

   Module :mod:`doctest`
      Tests embedded in documentation strings.


.. _writing-tests:

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__':
       unittest.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):
             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
  :class:`unittest.TestCase` are run as tests.  The :class:`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 :class:`unittest.TestCase`.


.. seealso::

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


.. _regrtest:

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:`test.support` --- Utilities for the Python test suite
===========================================================

.. module:: test.support
   :synopsis: Support for Python's regression test suite.


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

.. note::

   :mod:`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:: TestFailed

   Exception to be raised when a test fails. This is deprecated in favor of
   :mod:`unittest`\ -based tests and :class:`unittest.TestCase`'s assertion
   methods.


.. exception:: ResourceDenied

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


The :mod:`test.support` module defines the following constants:

.. data:: verbose

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


.. data:: is_jython

   ``True`` if the running interpreter is Jython.


.. data:: TESTFN

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

.. function:: forget(module_name)

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


.. function:: is_resource_enabled(resource)

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


.. function:: requires(resource, msg=None)

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


.. function:: 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.


.. function:: run_unittest(\*classes)

   Execute :class:`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 :func:`test_main` function::

      def test_main():
          support.run_unittest(__name__)

   This will run all tests defined in the named module.


.. function:: run_doctest(module, verbosity=None)

   Run :func:`doctest.testmod` on the given *module*.  Return
   ``(failure_count, test_count)``.

   If *verbosity* is ``None``, :func:`doctest.testmod` is run with verbosity
   set to :data:`verbose`.  Otherwise, it is run with verbosity set to
   ``None``.

.. function:: check_warnings(\*filters, quiet=True)

   A convenience wrapper for :func:`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
   :meth:`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 :class:`WarningRecorder` instance is
   returned. The underlying warnings list from
   :func:`~warnings.catch_warnings` is available via the recorder object's
   :attr:`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 :meth:`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, :func:`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:
          warnings.warn("foo")
          assert str(w.args[0]) == "foo"
          warnings.warn("bar")
          assert str(w.args[0]) == "bar"
          assert str(w.warnings[0].args[0]) == "foo"
          assert str(w.warnings[1].args[0]) == "bar"
          w.reset()
          assert len(w.warnings) == 0


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

   .. versionchanged:: 3.2
      New optional arguments *filters* and *quiet*.


.. function:: captured_stdin()
              captured_stdout()
              captured_stderr()

   A context managers that temporarily replaces the named stream with
   :class:`io.StringIO` object.

   Example use with output streams::

      with captured_stdout() as stdout, captured_stderr() as stderr:
          print("hello")
          print("error", file=sys.stderr)
      assert stdout.getvalue() == "hello\n"
      assert stderr.getvalue() == "error\n"

   Example use with input stream::

      with captured_stdin() as stdin:
          stdin.write('hello\n')
          stdin.seek(0)
          # call test code that consumes from sys.stdin
          captured = input()
      self.assertEqual(captured, "hello")


.. function:: temp_dir(path=None, quiet=False)

   A context manager that creates a temporary directory at *path* and
   yields the directory.

   If *path* is None, the temporary directory is created using
   :func:`tempfile.mkdtemp`.  If *quiet* is ``False``, the context manager
   raises an exception on error.  Otherwise, if *path* is specified and
   cannot be created, only a warning is issued.


.. function:: change_cwd(path, quiet=False)

   A context manager that temporarily changes the current working
   directory to *path* and yields the directory.

   If *quiet* is ``False``, the context manager raises an exception
   on error.  Otherwise, it issues only a warning and keeps the current
   working directory the same.


.. function:: temp_cwd(name='tempcwd', quiet=False)

   A context manager that temporarily creates a new directory and
   changes the current working directory (CWD).

   The context manager creates a temporary directory in the current
   directory with name *name* before temporarily changing the current
   working directory.  If *name* is None, the temporary directory is
   created using :func:`tempfile.mkdtemp`.

   If *quiet* is ``False`` and it is not possible to create or change
   the CWD, an error is raised.  Otherwise, only a warning is raised
   and the original CWD is used.


.. function:: temp_umask(umask)

   A context manager that temporarily sets the process umask.


.. function:: can_symlink()

   Return ``True`` if the OS supports symbolic links, ``False``
   otherwise.


.. decorator:: skip_unless_symlink()

   A decorator for running tests that require support for symbolic links.


.. decorator:: anticipate_failure(condition)

   A decorator to conditionally mark tests with
   :func:`unittest.expectedFailure`. Any use of this decorator should
   have an associated comment identifying the relevant tracker issue.


.. decorator:: run_with_locale(catstr, *locales)

   A decorator for running a function in a different locale, correctly
   resetting it after it has finished.  *catstr* is the locale category as
   a string (for example ``"LC_ALL"``).  The *locales* passed will be tried
   sequentially, and the first valid locale will be used.


.. function:: make_bad_fd()

   Create an invalid file descriptor by opening and closing a temporary file,
   and returning its descriptor.


.. function:: import_module(name, deprecated=False)

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

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

   .. versionadded:: 3.1


.. function:: 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 :func:`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 ``None``
   in the module cache during the import to ensure that attempts to import
   them raise :exc:`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 :exc:`ImportError` if 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'])

   .. versionadded:: 3.1


.. function:: bind_port(sock, host=HOST)

   Bind the socket to a free port and return the port number.  Relies on
   ephemeral ports in order to ensure we are using an unbound port.  This is
   important as many tests may be running simultaneously, especially in a
   buildbot environment.  This method raises an exception if the
   ``sock.family`` is :const:`~socket.AF_INET` and ``sock.type`` is
   :const:`~socket.SOCK_STREAM`, and the socket has
   :const:`~socket.SO_REUSEADDR` or :const:`~socket.SO_REUSEPORT` set on it.
   Tests should never set these socket options for TCP/IP sockets.
   The only case for setting these options is testing multicasting via
   multiple UDP sockets.

   Additionally, if the :const:`~socket.SO_EXCLUSIVEADDRUSE` socket option is
   available (i.e. on Windows), it will be set on the socket.  This will
   prevent anyone else from binding to our host/port for the duration of the
   test.


.. function:: find_unused_port(family=socket.AF_INET, socktype=socket.SOCK_STREAM)

   Returns an unused port that should be suitable for binding.  This is
   achieved by creating a temporary socket with the same family and type as
   the ``sock`` parameter (default is :const:`~socket.AF_INET`,
   :const:`~socket.SOCK_STREAM`),
   and binding it to the specified host address (defaults to ``0.0.0.0``)
   with the port set to 0, eliciting an unused ephemeral port from the OS.
   The temporary socket is then closed and deleted, and the ephemeral port is
   returned.

   Either this method or :func:`bind_port` should be used for any tests
   where a server socket needs to be bound to a particular port for the
   duration of the test.
   Which one to use depends on whether the calling code is creating a python
   socket, or if an unused port needs to be provided in a constructor
   or passed to an external program (i.e. the ``-accept`` argument to
   openssl's s_server mode).  Always prefer :func:`bind_port` over
   :func:`find_unused_port` where possible.  Using a hard coded port is
   discouraged since it can make multiple instances of the test impossible to
   run simultaneously, which is a problem for buildbots.


.. function:: load_package_tests(pkg_dir, loader, standard_tests, pattern)

   Generic implementation of the :mod:`unittest` ``load_tests`` protocol for
   use in test packages.  *pkg_dir* is the root directory of the package;
   *loader*, *standard_tests*, and *pattern* are the arguments expected by
   ``load_tests``.  In simple cases, the test package's ``__init__.py``
   can be the following::

      import os
      from test.support import load_package_tests

      def load_tests(*args):
          return load_package_tests(os.path.dirname(__file__), *args)

.. function:: detect_api_mismatch(ref_api, other_api, *, ignore=()):

   Returns the set of attributes, functions or methods of *ref_api* not
   found on *other_api*, except for a defined list of items to be
   ignored in this check specified in *ignore*.

   By default this skips private attributes beginning with '_' but
   includes all magic methods, i.e. those starting and ending in '__'.

   .. versionadded:: 3.5


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

.. class:: TransientResource(exc, **kwargs)

   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:: 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.

   .. versionchanged:: 3.1
      Added dictionary interface.

.. method:: EnvironmentVarGuard.set(envvar, value)

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


.. method:: EnvironmentVarGuard.unset(envvar)

   Temporarily unset the environment variable ``envvar``.


.. class:: SuppressCrashReport()

   A context manager used to try to prevent crash dialog popups on tests that
   are expected to crash a subprocess.

   On Windows, it disables Windows Error Reporting dialogs using
   `SetErrorMode <https://msdn.microsoft.com/en-us/library/windows/desktop/ms680621.aspx>`_.

   On UNIX, :func:`resource.setrlimit` is used to set
   :attr:`resource.RLIMIT_CORE`'s soft limit to 0 to prevent coredump file
   creation.

   On both platforms, the old value is restored by :meth:`__exit__`.


.. class:: WarningsRecorder()

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