26.8. "test" — Regression tests package for Python
**************************************************

Note: The "test" package is meant for internal use by Python only.
  It is documented for the benefit of the core developers of Python.
  Any use of this package outside of Python’s standard library is
  discouraged as code mentioned here can change or be removed without
  notice between releases of Python.

======================================================================

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

Each module in the "test" package whose name starts with "test_" is a
testing suite for a specific module or feature. All new tests should
be written using the "unittest" or "doctest" module.  Some older tests
are written using a “traditional” testing style that compares output
printed to "sys.stdout"; this style of test is considered deprecated.

See also:

  Module "unittest"
     Writing PyUnit regression tests.

  Module "doctest"
     Tests embedded in documentation strings.


26.8.1. 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 ...

   if __name__ == '__main__':
       unittest.main()

This code pattern allows the testing suite to be run by
"test.regrtest", on its own as a script that supports the "unittest"
CLI, or via the "python -m unittest" CLI.

The goal for regression testing is to try to break code. This leads to
a few guidelines to be followed:

* The testing suite should exercise all classes, functions, and
  constants. This includes not just the external API that is to be
  presented to the outside world but also “private” code.

* Whitebox testing (examining the code being tested when the tests
  are being written) is preferred. Blackbox testing (testing only the
  published user interface) is not complete enough to make sure all
  boundary and edge cases are tested.

* Make sure all possible values are tested including invalid ones.
  This makes sure that not only all valid values are acceptable but
  also that improper values are handled correctly.

* Exhaust as many code paths as possible. Test where branching
  occurs and thus tailor input to make sure as many different paths
  through the code are taken.

* Add an explicit test for any bugs discovered for the tested code.
  This will make sure that the error does not crop up again if the
  code is changed in the future.

* Make sure to clean up after your tests (such as close and remove
  all temporary files).

* If a test is dependent on a specific condition of the operating
  system then verify the condition already exists before attempting
  the test.

* Import as few modules as possible and do it as soon as possible.
  This minimizes external dependencies of tests and also minimizes
  possible anomalous behavior from side-effects of importing a module.

* Try to maximize code reuse. On occasion, tests will vary by
  something as small as what type of input is used. Minimize code
  duplication by subclassing a basic test class with a class that
  specifies the input:

     class TestFuncAcceptsSequencesMixin:

         func = mySuperWhammyFunction

         def test_func(self):
             self.func(self.arg)

     class AcceptLists(TestFuncAcceptsSequencesMixin, unittest.TestCase):
         arg = [1, 2, 3]

     class AcceptStrings(TestFuncAcceptsSequencesMixin, unittest.TestCase):
         arg = 'abc'

     class AcceptTuples(TestFuncAcceptsSequencesMixin, unittest.TestCase):
         arg = (1, 2, 3)

  When using this pattern, remember that all classes that inherit from
  "unittest.TestCase" are run as tests.  The "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 "unittest.TestCase".

See also:

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


26.8.2. Running tests using the command-line interface
======================================================

The "test" package can be run as a script to drive Python’s regression
test suite, thanks to the "-m" option: **python -m test**. Under the
hood, it uses "test.regrtest"; the call **python -m test.regrtest**
used in previous Python versions still works.  Running the script by
itself automatically starts running all regression tests in the "test"
package. It does this by finding all modules in the package whose name
starts with "test_", importing them, and executing the function
"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 (**python -m test test_spam**)
will minimize output and only print whether the test passed or failed.

Running "test" directly allows what resources are available for tests
to use to be set. You do this by using the "-u" command-line option.
Specifying "all" as the value for the "-u" option enables all possible
resources: **python -m test -uall**. If all but one resource is
desired (a more common case), a comma-separated list of resources that
are not desired may be listed after "all". The command **python -m
test -uall,-audio,-largefile** will run "test" with all resources
except the "audio" and "largefile" resources. For a list of all
resources and more command-line options, run **python -m test -h**.

Some other ways to execute the regression tests depend on what
platform the tests are being executed on. On Unix, you can run **make
test** at the top-level directory where Python was built. On Windows,
executing **rt.bat** from your "PCBuild" directory will run all
regression tests.


26.9. "test.support" — Utilities for the Python test suite
**********************************************************

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

Note: "test.support" is not a public module.  It is documented here
  to help Python developers write tests.  The API of this module is
  subject to change without backwards compatibility concerns between
  releases.

This module defines the following exceptions:

exception test.support.TestFailed

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

exception test.support.ResourceDenied

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

The "test.support" module defines the following constants:

test.support.verbose

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

test.support.is_jython

   "True" if the running interpreter is Jython.

test.support.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 "test.support" module defines the following functions:

test.support.forget(module_name)

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

test.support.is_resource_enabled(resource)

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

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

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

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

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

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

test.support.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.run_doctest(module, verbosity=None)

   Run "doctest.testmod()" on the given *module*.  Return
   "(failure_count, test_count)".

   If *verbosity* is "None", "doctest.testmod()" is run with verbosity
   set to "verbose".  Otherwise, it is run with verbosity set to
   "None".

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

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

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

   If no arguments are specified, it defaults to:

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

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

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

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

   The context manager is designed to be used like this:

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

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

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

      with check_warnings(quiet=True) as w:
          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.

   Changed in version 3.2: New optional arguments *filters* and
   *quiet*.

test.support.captured_stdin()
test.support.captured_stdout()
test.support.captured_stderr()

   A context managers that temporarily replaces the named stream with
   "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")

test.support.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
   "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.

test.support.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.

test.support.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 "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.

test.support.temp_umask(umask)

   A context manager that temporarily sets the process umask.

test.support.can_symlink()

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

@test.support.skip_unless_symlink

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

@test.support.anticipate_failure(condition)

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

@test.support.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.

test.support.make_bad_fd()

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

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
   "None" 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 "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'])

   New in version 3.1.

test.support.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 "AF_INET" and "sock.type" is
   "SOCK_STREAM", and the socket has "SO_REUSEADDR" or "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 "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.

test.support.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 "AF_INET", "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 "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 "bind_port()" over "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.

test.support.load_package_tests(pkg_dir, loader, standard_tests, pattern)

   Generic implementation of the "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)

test.support.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 ‘__’.

   New in version 3.5.

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.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.

   On UNIX, "resource.setrlimit()" is used to set
   "resource.RLIMIT_CORE"’s soft limit to 0 to prevent coredump file
   creation.

   On both platforms, the old value is restored by "__exit__()".

class test.support.WarningsRecorder

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