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

Starting with the 3.13 release, CPython has support for a build of
Python called *free threading* where the *global interpreter lock*
(GIL) is disabled.  Free-threaded execution allows for full
utilization of the available processing power by running threads in
parallel on available CPU cores. While not all software will benefit
from this automatically, programs designed with threading in mind will
run faster on multi-core hardware.

Some third-party packages, in particular ones with an *extension
module*, may not be ready for use in a free-threaded build, and will
re-enable the *GIL*.

This document describes the implications of free threading for Python
code.  See C API Extension Support for Free Threading for information
on how to write C extensions that support the free-threaded build.

See also:

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


Installation
============

Starting with Python 3.13, the official macOS and Windows installers
optionally support installing free-threaded Python binaries.  The
installers are available at https://www.python.org/downloads/.

For information on other platforms, see the Installing a Free-Threaded
Python, a community-maintained installation guide for installing free-
threaded Python.

When building CPython from source, the "--disable-gil" configure
option should be used to build a free-threaded Python interpreter.


Identifying free-threaded Python
================================

To check if the current interpreter supports free-threading, "python
-VV" and "sys.version" contain "free-threading build". The new
"sys._is_gil_enabled()" function can be used to check whether the GIL
is actually disabled in the running process.

The "sysconfig.get_config_var("Py_GIL_DISABLED")" configuration
variable can be used to determine whether the build supports free
threading.  If the variable is set to "1", then the build supports
free threading.  This is the recommended mechanism for decisions
related to the build configuration.


The global interpreter lock in free-threaded Python
===================================================

Free-threaded builds of CPython support optionally running with the
GIL enabled at runtime using the environment variable "PYTHON_GIL" or
the command-line option "-X gil".

The GIL may also automatically be enabled when importing a C-API
extension module that is not explicitly marked as supporting free
threading.  A warning will be printed in this case.

In addition to individual package documentation, the following
websites track the status of popular packages support for free
threading:

* https://py-free-threading.github.io/tracking/

* https://hugovk.github.io/free-threaded-wheels/


Thread safety
=============

The free-threaded build of CPython aims to provide similar thread-
safety behavior at the Python level to the default GIL-enabled build.
Built-in types like "dict", "list", and "set" use internal locks to
protect against concurrent modifications in ways that behave similarly
to the GIL.  However, Python has not historically guaranteed specific
behavior for concurrent modifications to these built-in types, so this
should be treated as a description of the current implementation, not
a guarantee of current or future behavior.

Note:

  It's recommended to use the "threading.Lock" or other
  synchronization primitives instead of relying on the internal locks
  of built-in types, when possible.


Known limitations
=================

This section describes known limitations of the free-threaded CPython
build.


Immortalization
---------------

In the free-threaded build, some objects are *immortal*. Immortal
objects are not deallocated and have reference counts that are never
modified.  This is done to avoid reference count contention that would
prevent efficient multi-threaded scaling.

As of the 3.14 release, immortalization is limited to:

* Code constants: numeric literals, string literals, and tuple
  literals composed of other constants.

* Strings interned by "sys.intern()".


Frame objects
-------------

It is not safe to access "frame.f_locals" from a frame object if that
frame is currently executing in another thread, and doing so may crash
the interpreter.


Iterators
---------

It is generally not thread-safe to access the same iterator object
from multiple threads concurrently, and threads may see duplicate or
missing elements.


Single-threaded performance
---------------------------

The free-threaded build has additional overhead when executing Python
code compared to the default GIL-enabled build.  The amount of
overhead depends on the workload and hardware.  On the pyperformance
benchmark suite, the average overhead ranges from about 1% on macOS
aarch64 to 8% on x86-64 Linux systems.


Behavioral changes
==================

This section describes CPython behavioural changes with the free-
threaded build.


Context variables
-----------------

In the free-threaded build, the flag "thread_inherit_context" is set
to true by default which causes threads created with
"threading.Thread" to start with a copy of the "Context()" of the
caller of "start()".  In the default GIL-enabled build, the flag
defaults to false so threads start with an empty "Context()".


Warning filters
---------------

In the free-threaded build, the flag "context_aware_warnings" is set
to true by default.  In the default GIL-enabled build, the flag
defaults to false.  If the flag is true then the
"warnings.catch_warnings" context manager uses a context variable for
warning filters.  If the flag is false then "catch_warnings" modifies
the global filters list, which is not thread-safe.  See the "warnings"
module for more details.
