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Recommend running the test suite in parallel #1917

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StanFromIreland merged 3 commits into from
Oct 27, 2026
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Recommend running the test suite in parallel #1917

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docs: recommend parallel test execution
harshit-s-gowda Oct 3, 2026
7ada3c7
Add section on running tests sequentially
StanFromIreland Oct 27, 2026
519d915
Apply Zach's suggestion
StanFromIreland Oct 27, 2026
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27 changes: 16 additions & 11 deletions testing/run-write-tests.rst
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Expand Up @@ -17,29 +17,29 @@ Running and writing tests
Running
=======

The shortest, simplest way of running the test suite is the following command
from the root directory of your checkout (after you have
:ref:`built Python <compiling>`):
The shortest, simplest way of running the test suite is the following command,
which runs tests in parallel, from the root directory of your checkout
(after you have :ref:`built Python <compiling>`):

.. tab:: Unix

.. code-block:: shell

./python -m test
./python -m test -j0

.. tab:: macOS

.. code-block:: shell

./python.exe -m test
./python.exe -m test -j0

This works on :ref:`most <mac-python.exe>` macOS systems.

.. tab:: Windows

.. code-block:: dosbatch

.\python.bat -m test
.\python.bat -m test -j0

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
Expand Down Expand Up @@ -139,26 +139,31 @@ But, there are several important notes:
In other words: if some module does not have ``unittest.main()``, then
most likely it does not support direct invocation.

If you have a multi-core or multi-CPU machine, you can enable parallel testing
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using several Python processes so as to speed up things:
By default, ``-j0`` runs tests in parallel using a number of worker processes
based on the CPU count, which is much faster but can interleave output
from different tests. To run tests sequentially, for example when debugging
a failure, drop the ``-j`` flag:

.. tab:: Unix

.. code-block:: shell

./python -m test -j0
./python -m test

.. tab:: macOS

.. code-block:: shell

./python.exe -m test -j0
./python.exe -m test

.. tab:: Windows

.. code-block:: dosbatch

.\python.bat -m test -j0
.\python.bat -m test

To limit how many tests run at once, pass an explicit number of
worker processes instead, such as ``-j2``.

.. _strenuous_testing:

Expand Down
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