sandbox/morph-devguide / runtests.rst

Running & Writing Tests


This document assumes you are working from an :ref:`in-development <indevbranch>` checkout of Python. If you are not then some things presented here may not work as they may depend on new features not available in earlier versions of Python.


The shortest, simplest way of running the test suite is the following command from the root directory of your checkout (after you have built Python):

./python -m test

If you don't have easy access to a command line, you can run the test suite from a Python or IDLE shell:

>>> from test import autotest

This will run the majority of tests, but exclude a small portion of them; these excluded tests use special kinds of resources: for example, accessing the Internet, or trying to play a sound or to display a graphical interface on your desktop. They are disabled by default so that running the test suite is not too intrusive. To enable some of these additional tests (and for other flags which can help debug various issues such as reference leaks), read the help text:

./python -m test -h

If you want to run a single test, simply specify the test name as an argument. You also probably want to enable verbose mode (using -v), so that individual failures are detailed:

./python -m test -v test_abc

If you have a multi-core or multi-CPU machine, you can enable parallel testing using several Python processes so as to speed up things:

./python -m test -j2

Finally, if you want to run tests under a more strenuous set of settings, you can run test as:

./python -bb -E -Wd -m test -r -w -uall

The various extra flags passed to Python cause it to be much stricter about various things (the -Wd flag should be -W error at some point, but the test suite has not reached a point where all warnings have been dealt with and so we cannot guarantee that a bug-free Python will properly complete a test run with -W error). The -r flag to the test runner causes it to run tests in a more random order which helps to check that the various tests do not interfere with each other. The -w flag causes failing tests to be run again to see if the failures are transient or consistent. The -uall flag allows the use of all available resources so as to not skip tests requiring, e.g., Internet access.


Writing tests for Python is much like writing tests for your own code. Tests need to be thorough, fast, isolated, consistently repeatable, and as simple as possible. We try to have tests both for normal behaviour and for error conditions. Tests live in the Lib/test directory, where every file that includes tests has a test_ prefix.

One difference with ordinary testing is that you are encouraged to rely on the :py:mod:`` module. It contains various helpers that are tailored to Python's test suite and help smooth out common problems such as platform differences, resource consumption and cleanup, or warnings management. That module is not suitable for use outside of the standard library.

When you are adding tests to an existing test file, it is also recommended that you study the other tests in that file; it will teach you which precautions you have to take to make your tests robust and portable.