test — Regression tests package for Python


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.

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__':

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

    When using this pattern, remember that all classes that inherit from unittest.TestCase are run as tests. The TestFuncAcceptsSequencesMixin class in the example above does not have any data and so can’t be run by itself, thus it does not inherit from unittest.TestCase.

See also

Test Driven Development

A book by Kent Beck on writing tests before code.

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.

test.support — Utilities for the Python test suite

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


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:


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.


True if the running interpreter is Jython.


True if the system is Android.


Path for shell if not on Windows; otherwise None.


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

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

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

Its default value is 5 seconds.



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 INTERNET_TIMEOUT should not mark a test as failed, but skip the test instead: see transient_internet().

Its default value is 1 minute.



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

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

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

Its default value is 30 seconds.


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.



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


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


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

Added in version 3.12.


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


Set to the top level directory that contains test.support.


Set to the top level directory for the test package.


Set to the data directory within the test package.


Set to sys.maxsize for big memory tests.


Set by set_memlimit() as the memory limit for big memory tests. Limited by MAX_Py_ssize_t.


Set by set_memlimit() as the memory limit for big memory tests. Not limited by MAX_Py_ssize_t.


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

See also the HAVE_DOCSTRINGS variable.


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

See also the MISSING_C_DOCSTRINGS variable.


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


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


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


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


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

The test.support module defines the following functions:

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

Run the loop body until break stops the loop.

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


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

Example of error=False usage:

for _ in support.busy_retry(support.SHORT_TIMEOUT, error=False):
    if check():
    raise RuntimeError('my custom error')
test.support.sleeping_retry(timeout, err_msg=None, /, *, init_delay=0.010, max_delay=1.0, error=True)

Wait strategy that applies exponential backoff.

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

See busy_retry() documentation for the parameters usage.

Example raising an exception after SHORT_TIMEOUT seconds:

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

Example of error=False usage:

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

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


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


Return _testcapi.WITH_PYMALLOC.

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.


Return a repr of dict with keys sorted.

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

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

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


Get size of a page in bytes.

Added in version 3.12.


Set the sys.setswitchinterval() to the given interval. Defines a minimum interval for Android systems to prevent the system from hanging.


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.

Set the values for max_memuse and real_max_memuse for big memory tests.


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


Return the original stdout set by record_original_stdout() or sys.stdout if it’s not set.


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


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


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("error", file=sys.stderr)
assert stdout.getvalue() == "hello\n"
assert stderr.getvalue() == "error\n"

Example use with input stream:

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

A context manager that temporary disables faulthandler.


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.


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

test.support.swap_attr(obj, attr, new_val)

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


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.

test.support.swap_item(obj, attr, new_val)

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


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.


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

Added in version 3.11.


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

Added in version 3.9.

test.support.wait_process(pid, *, exitcode, timeout=None)

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

Raise an AssertionError if the process exit code is not equal to exitcode.

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

Added in version 3.9.


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


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

test.support.checksizeof(test, o, size)

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


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


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

@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.


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


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


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


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


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


Decorator for skipping tests on non-IEEE 754 platforms.


Decorator for skipping tests if zlib doesn’t exist.


Decorator for skipping tests if gzip doesn’t exist.


Decorator for skipping tests if bz2 doesn’t exist.


Decorator for skipping tests if lzma doesn’t exist.


Decorator for skipping tests if resource is not available.


Decorator for only running the test if HAVE_DOCSTRINGS.


Decorator for only running the test if Limited C API is available.


Decorator for tests only applicable to CPython.

@test.support.impl_detail(msg=None, **guards)

Decorator for invoking check_impl_detail() on guards. If that returns False, then uses msg as the reason for skipping the test.


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

@test.support.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 for tests that fill the address space.

test.support.check_syntax_error(testcase, statement, errtext='', *, lineno=None, offset=None)

Test for syntax errors in statement by attempting to compile statement. testcase is the unittest instance for the test. errtext is the regular expression which should match the string representation of the raised 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.

test.support.open_urlresource(url, *args, **kw)

Open url. If open fails, raises TestFailed.


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.

test.support.get_attribute(obj, name)

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


Context manager catching unraisable exception using 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.


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)

Added in version 3.8.

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 ‘__’.

Added in version 3.5.

test.support.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.


Run code in subinterpreter. Raise unittest.SkipTest if tracemalloc is enabled.

test.support.check_free_after_iterating(test, iter, cls, args=())

Assert instances of cls are deallocated after iterating.


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.

test.support.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 __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)

Added in version 3.6.


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

Added in version 3.10.

test.support.check_disallow_instantiation(test_case, tp, *args, **kwds)

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

Added in version 3.10.


This function returns a context manager that will change the global 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.

Added in version 3.11.

The test.support module defines the following classes:

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.SaveSignals

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


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


Set the signal numbers from the save() dictionary to the saved handler.

class test.support.Matcher
matches(self, d, **kwargs)

Try to match a single dict with the supplied arguments.

match_value(self, k, dv, v)

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

test.support.socket_helper — Utilities for socket tests

The test.support.socket_helper module provides support for socket tests.

Added in version 3.9.


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

test.support.socket_helper.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 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.socket_helper.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.socket_helper.bind_unix_socket(sock, addr)

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


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

test.support.socket_helper.transient_internet(resource_name, *, timeout=30.0, errnos=())

A context manager that raises ResourceDenied when various issues with the internet connection manifest themselves as exceptions.

test.support.script_helper — Utilities for the Python execution tests

The test.support.script_helper module provides support for Python’s script execution tests.


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

test.support.script_helper.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.

Changed in version 3.9: The function no longer strips whitespaces from stderr.

test.support.script_helper.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.

Changed in version 3.9: The function no longer strips whitespaces from stderr.

test.support.script_helper.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 assert_python_ok() for more options.

Changed in version 3.9: The function no longer strips whitespaces from stderr.

test.support.script_helper.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 subprocess.Popen(). Returns a subprocess.Popen object.


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

test.support.script_helper.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.

test.support.script_helper.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).

test.support.script_helper.make_pkg(pkg_dir, init_source='')

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

test.support.script_helper.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.

test.support.bytecode_helper — Support tools for testing correct bytecode generation

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

Added in version 3.9.

The module defines the following class:

class test.support.bytecode_helper.BytecodeTestCase(unittest.TestCase)

This class has custom assertion methods for inspecting bytecode.


Return the disassembly of co as string.

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

Return instr if opname is found, otherwise throws AssertionError.

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

Throws AssertionError if opname is found.

test.support.threading_helper — Utilities for threading tests

The test.support.threading_helper module provides support for threading tests.

Added in version 3.10.

test.support.threading_helper.join_thread(thread, timeout=None)

Join a thread within timeout. Raise an AssertionError if thread is still alive after timeout seconds.


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

test.support.threading_helper.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 threading.Event.set(). start_threads will attempt to join the started threads upon exit.


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


Return current thread count and copy of dangling threads.


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


Context manager catching threading.Thread exception using threading.excepthook().

Attributes set when an exception is caught:

  • exc_type

  • exc_value

  • exc_traceback

  • thread

See threading.excepthook() documentation.

These attributes are deleted at the context manager exit.


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)

Added in version 3.8.

test.support.os_helper — Utilities for os tests

The test.support.os_helper module provides support for os tests.

Added in version 3.10.


A non-ASCII character encodable by os.fsencode().


Set to os.getcwd().


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).


Set to a filename containing the 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.


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.


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.


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

class test.support.os_helper.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.

class test.support.os_helper.FakePath(path)

Simple path-like object. It implements the __fspath__() method which just returns the path argument. If path is an exception, it will be raised in __fspath__().

EnvironmentVarGuard.set(envvar, value)

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


Temporarily unset the environment variable envvar.

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


Return True if the OS supports xattr, False otherwise.

test.support.os_helper.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.


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


Count the number of open file descriptors.


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


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


Call 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.


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

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


A decorator for running tests that require support for xattr.

test.support.os_helper.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.os_helper.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.


A context manager that temporarily sets the process umask.

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

test.support.import_helper — Utilities for import tests

The test.support.import_helper module provides support for import tests.

Added in version 3.10.


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

test.support.import_helper.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'])

Added in version 3.1.

test.support.import_helper.import_module(name, deprecated=False, *, required_on=())

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. 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 sys.platform.

Added in version 3.1.


Return a copy of sys.modules.


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


Delete name from sys.modules.


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 test.support.import_helper.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 DeprecationWarning on import. Example usage:

with CleanImport('foo'):
    importlib.import_module('foo')  # New reference.
class test.support.import_helper.DirsOnSysPath(*paths)

A context manager to temporarily add directories to sys.path.

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

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

test.support.warnings_helper — Utilities for warnings tests

The test.support.warnings_helper module provides support for warnings tests.

Added in version 3.10.

test.support.warnings_helper.ignore_warnings(*, category)

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

def test_suppress_warning():
    # do something

Added in version 3.8.


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

test.support.warnings_helper.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 SyntaxWarning is emitted only once, and that it will be converted to a SyntaxError when turned into error. testcase is the unittest instance for the test. errtext is the regular expression which should match the string representation of the emitted SyntaxWarning and raised 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.

Added in version 3.8.

test.support.warnings_helper.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:
    assert str(w.args[0]) == "foo"
    assert str(w.args[0]) == "bar"
    assert str(w.warnings[0].args[0]) == "foo"
    assert str(w.warnings[1].args[0]) == "bar"
    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.

class test.support.warnings_helper.WarningsRecorder

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