xref: /external/python/cpython3/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:`!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 :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
----------------------------------------------

.. module:: test.regrtest
   :synopsis: Drives the regression test suite.

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:: is_android

   ``True`` if the system is Android.


.. data:: unix_shell

   Path for shell if not on Windows; otherwise ``None``.


.. data:: 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 :meth:`~socket.socket.connect`,
   :meth:`~socket.socket.recv` and :meth:`~socket.socket.send` methods of
   :class:`socket.socket`.

   Its default value is 5 seconds.

   See also :data:`INTERNET_TIMEOUT`.


.. data:: INTERNET_TIMEOUT

   Timeout in seconds for network requests going to the internet.

   The timeout is short enough to prevent a test to wait for too long if the
   internet request is blocked for whatever reason.

   Usually, a timeout using :data:`INTERNET_TIMEOUT` should not mark a test as
   failed, but skip the test instead: see
   :func:`~test.support.socket_helper.transient_internet`.

   Its default value is 1 minute.

   See also :data:`LOOPBACK_TIMEOUT`.


.. data:: 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 :data:`SHORT_TIMEOUT` starts to fail randomly on slow
   buildbots, use :data:`LONG_TIMEOUT` instead.

   Its default value is 30 seconds.


.. data:: 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.

   See also :data:`LOOPBACK_TIMEOUT`, :data:`INTERNET_TIMEOUT` and
   :data:`SHORT_TIMEOUT`.


.. data:: PGO

   Set when tests can be skipped when they are not useful for PGO.


.. data:: PIPE_MAX_SIZE

   A constant that is likely larger than the underlying OS pipe buffer size,
   to make writes blocking.


.. data:: Py_DEBUG

   ``True`` if Python was built with the :c:macro:`Py_DEBUG` macro
   defined, that is, if
   Python was :ref:`built in debug mode <debug-build>`.

   .. versionadded:: 3.12


.. data:: SOCK_MAX_SIZE

   A constant that is likely larger than the underlying OS socket buffer size,
   to make writes blocking.


.. data:: TEST_SUPPORT_DIR

   Set to the top level directory that contains :mod:`test.support`.


.. data:: TEST_HOME_DIR

   Set to the top level directory for the test package.


.. data:: TEST_DATA_DIR

   Set to the ``data`` directory within the test package.


.. data:: MAX_Py_ssize_t

   Set to :data:`sys.maxsize` for big memory tests.


.. data:: max_memuse

   Set by :func:`set_memlimit` as the memory limit for big memory tests.
   Limited by :data:`MAX_Py_ssize_t`.


.. data:: real_max_memuse

   Set by :func:`set_memlimit` as the memory limit for big memory tests.  Not
   limited by :data:`MAX_Py_ssize_t`.


.. data:: MISSING_C_DOCSTRINGS

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

   See also the :data:`HAVE_DOCSTRINGS` variable.


.. data:: HAVE_DOCSTRINGS

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

   See also the :data:`MISSING_C_DOCSTRINGS` variable.


.. data:: TEST_HTTP_URL

   Define the URL of a dedicated HTTP server for the network tests.


.. data:: ALWAYS_EQ

   Object that is equal to anything.  Used to test mixed type comparison.


.. data:: NEVER_EQ

   Object that is not equal to anything (even to :data:`ALWAYS_EQ`).
   Used to test mixed type comparison.


.. data:: LARGEST

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


.. data:: SMALLEST

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


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

.. function:: busy_retry(timeout, err_msg=None, /, *, error=True)

   Run the loop body until ``break`` stops the loop.

   After *timeout* seconds, raise an :exc:`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')

.. function:: 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 :func:`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')

.. 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:: python_is_optimized()

   Return ``True`` if Python was not built with ``-O0`` or ``-Og``.


.. function:: with_pymalloc()

   Return :const:`_testcapi.WITH_PYMALLOC`.


.. 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:: sortdict(dict)

   Return a repr of *dict* with keys sorted.


.. 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:: get_pagesize()

   Get size of a page in bytes.

   .. versionadded:: 3.12


.. function:: setswitchinterval(interval)

   Set the :func:`sys.setswitchinterval` to the given *interval*.  Defines
   a minimum interval for Android systems to prevent the system from hanging.


.. function:: check_impl_detail(**guards)

   Use this check to guard CPython's implementation-specific tests or to
   run them only on the implementations guarded by the arguments.  This
   function returns ``True`` or ``False`` depending on the host platform.
   Example usage::

      check_impl_detail()               # Only on CPython (default).
      check_impl_detail(jython=True)    # Only on Jython.
      check_impl_detail(cpython=False)  # Everywhere except CPython.


.. function:: set_memlimit(limit)

   Set the values for :data:`max_memuse` and :data:`real_max_memuse` for big
   memory tests.


.. function:: record_original_stdout(stdout)

   Store the value from *stdout*.  It is meant to hold the stdout at the
   time the regrtest began.


.. function:: get_original_stdout()

   Return the original stdout set by :func:`record_original_stdout` or
   ``sys.stdout`` if it's not set.


.. function:: args_from_interpreter_flags()

   Return a list of command line arguments reproducing the current settings
   in ``sys.flags`` and ``sys.warnoptions``.


.. function:: optim_args_from_interpreter_flags()

   Return a list of command line arguments reproducing the current
   optimization settings in ``sys.flags``.


.. 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:: disable_faulthandler()

   A context manager that temporary disables :mod:`faulthandler`.


.. function:: gc_collect()

   Force as many objects as possible to be collected.  This is needed because
   timely deallocation is not guaranteed by the garbage collector.  This means
   that ``__del__`` methods may be called later than expected and weakrefs
   may remain alive for longer than expected.


.. function:: disable_gc()

   A context manager that disables the garbage collector on entry. On
   exit, the garbage collector is restored to its prior state.


.. function:: swap_attr(obj, attr, new_val)

   Context manager to swap out an attribute with a new object.

   Usage::

      with swap_attr(obj, "attr", 5):
          ...

   This will set ``obj.attr`` to 5 for the duration of the ``with`` block,
   restoring the old value at the end of the block.  If ``attr`` doesn't
   exist on ``obj``, it will be created and then deleted at the end of the
   block.

   The old value (or ``None`` if it doesn't exist) will be assigned to the
   target of the "as" clause, if there is one.


.. function:: swap_item(obj, attr, new_val)

   Context manager to swap out an item with a new object.

   Usage::

      with swap_item(obj, "item", 5):
          ...

   This will set ``obj["item"]`` to 5 for the duration of the ``with`` block,
   restoring the old value at the end of the block. If ``item`` doesn't
   exist on ``obj``, it will be created and then deleted at the end of the
   block.

   The old value (or ``None`` if it doesn't exist) will be assigned to the
   target of the "as" clause, if there is one.


.. function:: flush_std_streams()

   Call the ``flush()`` method on :data:`sys.stdout` and then on
   :data:`sys.stderr`. It can be used to make sure that the logs order is
   consistent before writing into stderr.

   .. versionadded:: 3.11


.. function:: print_warning(msg)

   Print a warning into :data:`sys.__stderr__`. Format the message as:
   ``f"Warning -- {msg}"``. If *msg* is made of multiple lines, add
   ``"Warning -- "`` prefix to each line.

   .. versionadded:: 3.9


.. function:: wait_process(pid, *, exitcode, timeout=None)

   Wait until process *pid* completes and check that the process exit code is
   *exitcode*.

   Raise an :exc:`AssertionError` if the process exit code is not equal to
   *exitcode*.

   If the process runs longer than *timeout* seconds (:data:`SHORT_TIMEOUT` by
   default), kill the process and raise an :exc:`AssertionError`. The timeout
   feature is not available on Windows.

   .. versionadded:: 3.9


.. function:: calcobjsize(fmt)

   Return the size of the :c:type:`PyObject` whose structure members are
   defined by *fmt*. The returned value includes the size of the Python object header and alignment.


.. function:: calcvobjsize(fmt)

   Return the size of the :c:type:`PyVarObject` whose structure members are
   defined by *fmt*. The returned value includes the size of the Python object header and alignment.


.. function:: checksizeof(test, o, size)

   For testcase *test*, assert that the ``sys.getsizeof`` for *o* plus the GC
   header size equals *size*.


.. 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.


.. function:: system_must_validate_cert(f)

   A decorator that skips the decorated test on TLS certification validation failures.


.. 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.


.. decorator:: run_with_tz(tz)

   A decorator for running a function in a specific timezone, correctly
   resetting it after it has finished.


.. decorator:: requires_freebsd_version(*min_version)

   Decorator for the minimum version when running test on FreeBSD.  If the
   FreeBSD version is less than the minimum, the test is skipped.


.. decorator:: requires_linux_version(*min_version)

   Decorator for the minimum version when running test on Linux.  If the
   Linux version is less than the minimum, the test is skipped.


.. decorator:: requires_mac_version(*min_version)

   Decorator for the minimum version when running test on macOS.  If the
   macOS version is less than the minimum, the test is skipped.


.. decorator:: requires_gil_enabled

   Decorator for skipping tests on the free-threaded build.  If the
   :term:`GIL` is disabled, the test is skipped.


.. decorator:: requires_IEEE_754

   Decorator for skipping tests on non-IEEE 754 platforms.


.. decorator:: requires_zlib

   Decorator for skipping tests if :mod:`zlib` doesn't exist.


.. decorator:: requires_gzip

   Decorator for skipping tests if :mod:`gzip` doesn't exist.


.. decorator:: requires_bz2

   Decorator for skipping tests if :mod:`bz2` doesn't exist.


.. decorator:: requires_lzma

   Decorator for skipping tests if :mod:`lzma` doesn't exist.


.. decorator:: requires_resource(resource)

   Decorator for skipping tests if *resource* is not available.


.. decorator:: requires_docstrings

   Decorator for only running the test if :data:`HAVE_DOCSTRINGS`.


.. decorator:: requires_limited_api

   Decorator for only running the test if :ref:`Limited C API <limited-c-api>`
   is available.


.. decorator:: cpython_only

   Decorator for tests only applicable to CPython.


.. decorator:: impl_detail(msg=None, **guards)

   Decorator for invoking :func:`check_impl_detail` on *guards*.  If that
   returns ``False``, then uses *msg* as the reason for skipping the test.


.. decorator:: no_tracing

   Decorator to temporarily turn off tracing for the duration of the test.


.. decorator:: refcount_test

   Decorator for tests which involve reference counting.  The decorator does
   not run the test if it is not run by CPython.  Any trace function is unset
   for the duration of the test to prevent unexpected refcounts caused by
   the trace function.


.. decorator:: bigmemtest(size, memuse, dry_run=True)

   Decorator for bigmem tests.

   *size* is a requested size for the test (in arbitrary, test-interpreted
   units.)  *memuse* is the number of bytes per unit for the test, or a good
   estimate of it.  For example, a test that needs two byte buffers, of 4 GiB
   each, could be decorated with ``@bigmemtest(size=_4G, memuse=2)``.

   The *size* argument is normally passed to the decorated test method as an
   extra argument.  If *dry_run* is ``True``, the value passed to the test
   method may be less than the requested value.  If *dry_run* is ``False``, it
   means the test doesn't support dummy runs when ``-M`` is not specified.


.. decorator:: bigaddrspacetest

   Decorator for tests that fill the address space.


.. function:: check_syntax_error(testcase, statement, errtext='', *, lineno=None, offset=None)

   Test for syntax errors in *statement* by attempting to compile *statement*.
   *testcase* is the :mod:`unittest` instance for the test.  *errtext* is the
   regular expression which should match the string representation of the
   raised :exc:`SyntaxError`.  If *lineno* is not ``None``, compares to
   the line of the exception.  If *offset* is not ``None``, compares to
   the offset of the exception.


.. function:: open_urlresource(url, *args, **kw)

   Open *url*.  If open fails, raises :exc:`TestFailed`.


.. function:: reap_children()

   Use this at the end of ``test_main`` whenever sub-processes are started.
   This will help ensure that no extra children (zombies) stick around to
   hog resources and create problems when looking for refleaks.


.. function:: get_attribute(obj, name)

   Get an attribute, raising :exc:`unittest.SkipTest` if :exc:`AttributeError`
   is raised.


.. function:: catch_unraisable_exception()

   Context manager catching unraisable exception using
   :func:`sys.unraisablehook`.

   Storing the exception value (``cm.unraisable.exc_value``) creates a
   reference cycle. The reference cycle is broken explicitly when the context
   manager exits.

   Storing the object (``cm.unraisable.object``) can resurrect it if it is set
   to an object which is being finalized. Exiting the context manager clears
   the stored object.

   Usage::

       with support.catch_unraisable_exception() as cm:
           # code creating an "unraisable exception"
           ...

           # check the unraisable exception: use cm.unraisable
           ...

       # cm.unraisable attribute no longer exists at this point
       # (to break a reference cycle)

   .. versionadded:: 3.8


.. 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


.. function:: patch(test_instance, object_to_patch, attr_name, new_value)

   Override *object_to_patch.attr_name* with *new_value*.  Also add
   cleanup procedure to *test_instance* to restore *object_to_patch* for
   *attr_name*.  The *attr_name* should be a valid attribute for
   *object_to_patch*.


.. function:: run_in_subinterp(code)

   Run *code* in subinterpreter.  Raise :exc:`unittest.SkipTest` if
   :mod:`tracemalloc` is enabled.


.. function:: check_free_after_iterating(test, iter, cls, args=())

   Assert instances of *cls* are deallocated after iterating.


.. function:: missing_compiler_executable(cmd_names=[])

   Check for the existence of the compiler executables whose names are listed
   in *cmd_names* or all the compiler executables when *cmd_names* is empty
   and return the first missing executable or ``None`` when none is found
   missing.


.. function:: check__all__(test_case, module, name_of_module=None, extra=(), not_exported=())

   Assert that the ``__all__`` variable of *module* contains all public names.

   The module's public names (its API) are detected automatically
   based on whether they match the public name convention and were defined in
   *module*.

   The *name_of_module* argument can specify (as a string or tuple thereof) what
   module(s) an API could be defined in order to be detected as a public
   API. One case for this is when *module* imports part of its public API from
   other modules, possibly a C backend (like ``csv`` and its ``_csv``).

   The *extra* argument can be a set of names that wouldn't otherwise be automatically
   detected as "public", like objects without a proper :attr:`~definition.__module__`
   attribute. If provided, it will be added to the automatically detected ones.

   The *not_exported* argument can be a set of names that must not be treated
   as part of the public API even though their names indicate otherwise.

   Example use::

      import bar
      import foo
      import unittest
      from test import support

      class MiscTestCase(unittest.TestCase):
          def test__all__(self):
              support.check__all__(self, foo)

      class OtherTestCase(unittest.TestCase):
          def test__all__(self):
              extra = {'BAR_CONST', 'FOO_CONST'}
              not_exported = {'baz'}  # Undocumented name.
              # bar imports part of its API from _bar.
              support.check__all__(self, bar, ('bar', '_bar'),
                                   extra=extra, not_exported=not_exported)

   .. versionadded:: 3.6

.. function:: skip_if_broken_multiprocessing_synchronize()

   Skip tests if the :mod:`multiprocessing.synchronize` module is missing, if
   there is no available semaphore implementation, or if creating a lock raises
   an :exc:`OSError`.

   .. versionadded:: 3.10


.. function:: check_disallow_instantiation(test_case, tp, *args, **kwds)

   Assert that type *tp* cannot be instantiated using *args* and *kwds*.

   .. versionadded:: 3.10


.. function:: adjust_int_max_str_digits(max_digits)

   This function returns a context manager that will change the global
   :func:`sys.set_int_max_str_digits` setting for the duration of the
   context to allow execution of test code that needs a different limit
   on the number of digits when converting between an integer and string.

   .. versionadded:: 3.11


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


.. 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
   :const:`resource.RLIMIT_CORE`'s soft limit to 0 to prevent coredump file
   creation.

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


.. class:: SaveSignals()

   Class to save and restore signal handlers registered by the Python signal
   handler.

   .. method:: save(self)

      Save the signal handlers to a dictionary mapping signal numbers to the
      current signal handler.

   .. method:: restore(self)

      Set the signal numbers from the :meth:`save` dictionary to the saved
      handler.


.. class:: Matcher()

   .. method:: matches(self, d, **kwargs)

      Try to match a single dict with the supplied arguments.


   .. method:: match_value(self, k, dv, v)

      Try to match a single stored value (*dv*) with a supplied value (*v*).


:mod:`test.support.socket_helper` --- Utilities for socket tests
================================================================

.. module:: test.support.socket_helper
   :synopsis: Support for socket tests.


The :mod:`test.support.socket_helper` module provides support for socket tests.

.. versionadded:: 3.9


.. data:: IPV6_ENABLED

    Set to ``True`` if IPv6 is enabled on this host, ``False`` otherwise.


.. 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:: 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:: bind_unix_socket(sock, addr)

   Bind a Unix socket, raising :exc:`unittest.SkipTest` if
   :exc:`PermissionError` is raised.


.. decorator:: skip_unless_bind_unix_socket

   A decorator for running tests that require a functional ``bind()`` for Unix
   sockets.


.. function:: transient_internet(resource_name, *, timeout=30.0, errnos=())

   A context manager that raises :exc:`~test.support.ResourceDenied` when
   various issues with the internet connection manifest themselves as
   exceptions.


:mod:`test.support.script_helper` --- Utilities for the Python execution tests
==============================================================================

.. module:: test.support.script_helper
   :synopsis: Support for Python's script execution tests.


The :mod:`test.support.script_helper` module provides support for Python's
script execution tests.

.. function:: interpreter_requires_environment()

   Return ``True`` if ``sys.executable interpreter`` requires environment
   variables in order to be able to run at all.

   This is designed to be used with ``@unittest.skipIf()`` to annotate tests
   that need to use an ``assert_python*()`` function to launch an isolated
   mode (``-I``) or no environment mode (``-E``) sub-interpreter process.

   A normal build & test does not run into this situation but it can happen
   when trying to run the standard library test suite from an interpreter that
   doesn't have an obvious home with Python's current home finding logic.

   Setting :envvar:`PYTHONHOME` is one way to get most of the testsuite to run
   in that situation.  :envvar:`PYTHONPATH` or :envvar:`PYTHONUSERSITE` are
   other common environment variables that might impact whether or not the
   interpreter can start.


.. function:: run_python_until_end(*args, **env_vars)

   Set up the environment based on *env_vars* for running the interpreter
   in a subprocess.  The values can include ``__isolated``, ``__cleanenv``,
   ``__cwd``, and ``TERM``.

   .. versionchanged:: 3.9
      The function no longer strips whitespaces from *stderr*.


.. function:: assert_python_ok(*args, **env_vars)

   Assert that running the interpreter with *args* and optional environment
   variables *env_vars* succeeds (``rc == 0``) and return a ``(return code,
   stdout, stderr)`` tuple.

   If the *__cleanenv* keyword-only parameter is set, *env_vars* is used as a fresh
   environment.

   Python is started in isolated mode (command line option ``-I``),
   except if the *__isolated* keyword-only parameter is set to ``False``.

   .. versionchanged:: 3.9
      The function no longer strips whitespaces from *stderr*.


.. function:: assert_python_failure(*args, **env_vars)

   Assert that running the interpreter with *args* and optional environment
   variables *env_vars* fails (``rc != 0``) and return a ``(return code,
   stdout, stderr)`` tuple.

   See :func:`assert_python_ok` for more options.

   .. versionchanged:: 3.9
      The function no longer strips whitespaces from *stderr*.


.. function:: spawn_python(*args, stdout=subprocess.PIPE, stderr=subprocess.STDOUT, **kw)

   Run a Python subprocess with the given arguments.

   *kw* is extra keyword args to pass to :func:`subprocess.Popen`. Returns a
   :class:`subprocess.Popen` object.


.. function:: kill_python(p)

   Run the given :class:`subprocess.Popen` process until completion and return
   stdout.


.. function:: make_script(script_dir, script_basename, source, omit_suffix=False)

   Create script containing *source* in path *script_dir* and *script_basename*.
   If *omit_suffix* is ``False``, append ``.py`` to the name.  Return the full
   script path.


.. function:: make_zip_script(zip_dir, zip_basename, script_name, name_in_zip=None)

   Create zip file at *zip_dir* and *zip_basename* with extension ``zip`` which
   contains the files in *script_name*. *name_in_zip* is the archive name.
   Return a tuple containing ``(full path, full path of archive name)``.


.. function:: make_pkg(pkg_dir, init_source='')

   Create a directory named *pkg_dir* containing an ``__init__`` file with
   *init_source* as its contents.


.. function:: make_zip_pkg(zip_dir, zip_basename, pkg_name, script_basename, \
                           source, depth=1, compiled=False)

   Create a zip package directory with a path of *zip_dir* and *zip_basename*
   containing an empty ``__init__`` file and a file *script_basename*
   containing the *source*.  If *compiled* is ``True``, both source files will
   be compiled and added to the zip package.  Return a tuple of the full zip
   path and the archive name for the zip file.


:mod:`test.support.bytecode_helper` --- Support tools for testing correct bytecode generation
=============================================================================================

.. module:: test.support.bytecode_helper
   :synopsis: Support tools for testing correct bytecode generation.

The :mod:`test.support.bytecode_helper` module provides support for testing
and inspecting bytecode generation.

.. versionadded:: 3.9

The module defines the following class:

.. class:: BytecodeTestCase(unittest.TestCase)

   This class has custom assertion methods for inspecting bytecode.

.. method:: BytecodeTestCase.get_disassembly_as_string(co)

   Return the disassembly of *co* as string.


.. method:: BytecodeTestCase.assertInBytecode(x, opname, argval=_UNSPECIFIED)

   Return instr if *opname* is found, otherwise throws :exc:`AssertionError`.


.. method:: BytecodeTestCase.assertNotInBytecode(x, opname, argval=_UNSPECIFIED)

   Throws :exc:`AssertionError` if *opname* is found.


:mod:`test.support.threading_helper` --- Utilities for threading tests
======================================================================

.. module:: test.support.threading_helper
   :synopsis: Support for threading tests.

The :mod:`test.support.threading_helper` module provides support for threading tests.

.. versionadded:: 3.10


.. function:: join_thread(thread, timeout=None)

   Join a *thread* within *timeout*.  Raise an :exc:`AssertionError` if thread
   is still alive after *timeout* seconds.


.. decorator:: reap_threads

   Decorator to ensure the threads are cleaned up even if the test fails.


.. function:: start_threads(threads, unlock=None)

   Context manager to start *threads*, which is a sequence of threads.
   *unlock* is a function called after the threads are started, even if an
   exception was raised; an example would be :meth:`threading.Event.set`.
   ``start_threads`` will attempt to join the started threads upon exit.


.. function:: threading_cleanup(*original_values)

   Cleanup up threads not specified in *original_values*.  Designed to emit
   a warning if a test leaves running threads in the background.


.. function:: threading_setup()

   Return current thread count and copy of dangling threads.


.. function:: wait_threads_exit(timeout=None)

   Context manager to wait until all threads created in the ``with`` statement
   exit.


.. function:: catch_threading_exception()

   Context manager catching :class:`threading.Thread` exception using
   :func:`threading.excepthook`.

   Attributes set when an exception is caught:

   * ``exc_type``
   * ``exc_value``
   * ``exc_traceback``
   * ``thread``

   See :func:`threading.excepthook` documentation.

   These attributes are deleted at the context manager exit.

   Usage::

       with threading_helper.catch_threading_exception() as cm:
           # code spawning a thread which raises an exception
           ...

           # check the thread exception, use cm attributes:
           # exc_type, exc_value, exc_traceback, thread
           ...

       # exc_type, exc_value, exc_traceback, thread attributes of cm no longer
       # exists at this point
       # (to avoid reference cycles)

   .. versionadded:: 3.8


:mod:`test.support.os_helper` --- Utilities for os tests
========================================================================

.. module:: test.support.os_helper
   :synopsis: Support for os tests.

The :mod:`test.support.os_helper` module provides support for os tests.

.. versionadded:: 3.10


.. data:: FS_NONASCII

   A non-ASCII character encodable by :func:`os.fsencode`.


.. data:: SAVEDCWD

   Set to :func:`os.getcwd`.


.. 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).


.. data:: TESTFN_NONASCII

   Set to a filename containing the :data:`FS_NONASCII` character, if it exists.
   This guarantees that if the filename exists, it can be encoded and decoded
   with the default filesystem encoding. This allows tests that require a
   non-ASCII filename to be easily skipped on platforms where they can't work.


.. data:: TESTFN_UNENCODABLE

   Set to a filename (str type) that should not be able to be encoded by file
   system encoding in strict mode.  It may be ``None`` if it's not possible to
   generate such a filename.


.. data:: TESTFN_UNDECODABLE

   Set to a filename (bytes type) that should not be able to be decoded by
   file system encoding in strict mode.  It may be ``None`` if it's not
   possible to generate such a filename.


.. data:: TESTFN_UNICODE

    Set to a non-ASCII name for a temporary file.


.. 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.


.. class:: FakePath(path)

   Simple :term:`path-like object`.  It implements the
   :meth:`~os.PathLike.__fspath__`
   method which just returns the *path* argument.  If *path* is an exception,
   it will be raised in :meth:`!__fspath__`.


.. 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``.


.. function:: can_symlink()

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


.. function:: can_xattr()

   Return ``True`` if the OS supports xattr, ``False``
   otherwise.


.. 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:: create_empty_file(filename)

   Create an empty file with *filename*.  If it already exists, truncate it.


.. function:: fd_count()

   Count the number of open file descriptors.


.. function:: fs_is_case_insensitive(directory)

   Return ``True`` if the file system for *directory* is case-insensitive.


.. function:: make_bad_fd()

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


.. function:: rmdir(filename)

   Call :func:`os.rmdir` on *filename*.  On Windows platforms, this is
   wrapped with a wait loop that checks for the existence of the file,
   which is needed due to antivirus programs that can hold files open and prevent
   deletion.


.. function:: rmtree(path)

   Call :func:`shutil.rmtree` on *path* or call :func:`os.lstat` and
   :func:`os.rmdir` to remove a path and its contents.  As with :func:`rmdir`,
   on Windows platforms
   this is wrapped with a wait loop that checks for the existence of the files.


.. decorator:: skip_unless_symlink

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


.. decorator:: skip_unless_xattr

   A decorator for running tests that require support for xattr.


.. 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_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:: temp_umask(umask)

   A context manager that temporarily sets the process umask.


.. function:: unlink(filename)

   Call :func:`os.unlink` on *filename*.  As with :func:`rmdir`,
   on Windows platforms, this is
   wrapped with a wait loop that checks for the existence of the file.


:mod:`test.support.import_helper` --- Utilities for import tests
================================================================

.. module:: test.support.import_helper
   :synopsis: Support for import tests.

The :mod:`test.support.import_helper` module provides support for import tests.

.. versionadded:: 3.10


.. function:: forget(module_name)

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


.. 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:: import_module(name, deprecated=False, *, required_on=())

   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``.  If a module is required on a platform but
   optional for others, set *required_on* to an iterable of platform prefixes
   which will be compared against :data:`sys.platform`.

   .. versionadded:: 3.1


.. function:: modules_setup()

   Return a copy of :data:`sys.modules`.


.. function:: modules_cleanup(oldmodules)

   Remove modules except for *oldmodules* and ``encodings`` in order to
   preserve internal cache.


.. function:: unload(name)

   Delete *name* from ``sys.modules``.


.. function:: make_legacy_pyc(source)

   Move a :pep:`3147`/:pep:`488` pyc file to its legacy pyc location and return the file
   system path to the legacy pyc file.  The *source* value is the file system
   path to the source file.  It does not need to exist, however the PEP
   3147/488 pyc file must exist.


.. class:: CleanImport(*module_names)

   A context manager to force import to return a new module reference.  This
   is useful for testing module-level behaviors, such as the emission of a
   :exc:`DeprecationWarning` on import.  Example usage::

      with CleanImport('foo'):
          importlib.import_module('foo')  # New reference.


.. class:: DirsOnSysPath(*paths)

   A context manager to temporarily add directories to :data:`sys.path`.

   This makes a copy of :data:`sys.path`, appends any directories given
   as positional arguments, then reverts :data:`sys.path` to the copied
   settings when the context ends.

   Note that *all* :data:`sys.path` modifications in the body of the
   context manager, including replacement of the object,
   will be reverted at the end of the block.


:mod:`test.support.warnings_helper` --- Utilities for warnings tests
====================================================================

.. module:: test.support.warnings_helper
   :synopsis: Support for warnings tests.

The :mod:`test.support.warnings_helper` module provides support for warnings tests.

.. versionadded:: 3.10


.. function:: ignore_warnings(*, category)

   Suppress warnings that are instances of *category*,
   which must be :exc:`Warning` or a subclass.
   Roughly equivalent to :func:`warnings.catch_warnings`
   with :meth:`warnings.simplefilter('ignore', category=category) <warnings.simplefilter>`.
   For example::

      @warning_helper.ignore_warnings(category=DeprecationWarning)
      def test_suppress_warning():
          # do something

   .. versionadded:: 3.8


.. function:: check_no_resource_warning(testcase)

   Context manager to check that no :exc:`ResourceWarning` was raised.  You
   must remove the object which may emit :exc:`ResourceWarning` before the
   end of the context manager.


.. function:: check_syntax_warning(testcase, statement, errtext='', *, lineno=1, offset=None)

   Test for syntax warning in *statement* by attempting to compile *statement*.
   Test also that the :exc:`SyntaxWarning` is emitted only once, and that it
   will be converted to a :exc:`SyntaxError` when turned into error.
   *testcase* is the :mod:`unittest` instance for the test.  *errtext* is the
   regular expression which should match the string representation of the
   emitted :exc:`SyntaxWarning` and raised :exc:`SyntaxError`.  If *lineno*
   is not ``None``, compares to the line of the warning and exception.
   If *offset* is not ``None``, compares to the offset of the exception.

   .. versionadded:: 3.8


.. 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*.


.. class:: WarningsRecorder()

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