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gh-124370: Add "howto" for free-threaded Python #124371

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gh-124370: Add "howto" for free-threaded Python
colesbury Jun 21, 2024
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@colesbury
colesbury committed Sep 24, 2024
commit 3b173e0d648cc447e82d6fc120f458552ffca62d
25 changes: 17 additions & 8 deletions Doc/howto/free-threading-python.rst
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@@ -1,15 +1,20 @@
.. _freethreading-python-howto:

*********************************
Python support for free threading
*********************************
**********************************************
Python experimental support for free threading
**********************************************

Starting with the 3.13 release, CPython has experimental support for running
with the :term:`global interpreter lock` (GIL) disabled in a configuration
with the :term:`global interpreter lock` (GIL) disabled in a build of Python
called :term:`free threading`. This document describes the implications of
free threading for Python code. See :ref:`freethreading-extensions-howto` for
information on how to write C extensions that support the free-threaded build.
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We seem to switch between "free threading" and "free-threaded" kind of randomly. I can't decide if this is OK or if we should choose one and stick with it.

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I think it's fine. We also say both "multithreaded" and "multithreading" depending on the context.


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I wonder if we could add a quick snapshot of the overall plan: if everything works out, eventually free threading will be the only build, etc. Also, maybe a statement about how most programmer won't need to be concerned with this, we're doing a lot to keep everyday Python programs behaving the same, etc.

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I would perhaps add a note in the seealso block that refers to the PEP and adds 1 or 2 highlights:

  • performance conscious users may wish to try the free threaded version
  • most users can safely continue to use the default CPython version
  • when free threading is no longer experimental, the versions will converge into one version for all.

.. seealso::

:pep:`703` – Making the Global Interpreter Lock Optional in CPython for an
overall description of free-threaded Python.


Installation
============
Expand All @@ -31,7 +36,7 @@ Identifying free-threaded Python

The free-threaded build of CPython can optionally run with the global
interpreter lock enabled, such as when :envvar:`PYTHON_GIL` is set to ``1``,
or when importing an extension module that requires the GIL.
or automatically when importing an extension module that requires the GIL.

The :func:`sys._is_gil_enabled` function will return ``False`` if the global
interpreter lock is currently disabled. This is the recommended mechanism for
Expand Down Expand Up @@ -97,8 +102,11 @@ Frame objects
-------------

It is not safe to access :ref:`frame <frame-objects>` objects from other
threads. This means that :func:`sys._current_frames` is generally not safe to
use in a free-threaded build.
threads and doing so may cause your program to crash . This means that
:func:`sys._current_frames` is generally not safe to use in a free-threaded
build. Functions like :func:`inspect.currentframe` and :func:`sys._getframe`
are generally safe as long as the resulting frame object is not passed to
another thread.

Iterators
---------
Expand All @@ -116,4 +124,5 @@ compared to the default GIL-enabled build. In 3.13, this overhead is about
40% on the `pyperformance <https://pyperformance.readthedocs.io/>`_ suite.
Programs that spend most of their time in C extensions or I/O will see
less of an impact. This overhead is expected to be reduced in Python
3.14.
3.14. We are aiming for an overhead of 10% or less on the pyperformance
suite compared to the default GIL-enabled build.
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