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[3.14] Add whatsnew text for warnings module changes. (gh-135869) #135961

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Jun 26, 2025
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[3.14] Add whatsnew text for warnings module changes. (gh-135869) #135961

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29 changes: 29 additions & 0 deletions Doc/whatsnew/3.14.rst
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Expand Up @@ -978,6 +978,23 @@ be specified by the build backend, as it will no longer be determined
automatically by the C compiler. For a running interpreter, the setting that
was used at compile time can be found using :func:`sysconfig.get_config_var`.

A new flag has been added, :data:`~sys.flags.context_aware_warnings`. This
flag defaults to true for the free-threaded build and false for the GIL-enabled
build. If the flag is true then the :class:`warnings.catch_warnings` context
manager uses a context variable for warning filters. This makes the context
manager behave predicably when used with multiple threads or asynchronous
tasks.

A new flag has been added, :data:`~sys.flags.thread_inherit_context`. This flag
defaults to true for the free-threaded build and false for the GIL-enabled
build. If the flag is true then threads created with :class:`threading.Thread`
start with a copy of the :class:`~contextvars.Context()` of the caller of
:meth:`~threading.Thread.start`. Most significantly, this makes the warning
filtering context established by :class:`~warnings.catch_warnings` be
"inherited" by threads (or asyncio tasks) started within that context. It also
affects other modules that use context variables, such as the :mod:`decimal`
context manager.


.. _whatsnew314-pyrepl-highlighting:

Expand Down Expand Up @@ -1028,6 +1045,18 @@ Please report any bugs or major performance regressions that you encounter!

.. seealso:: :pep:`744`

Concurrent safe warnings control
--------------------------------

The :class:`warnings.catch_warnings` context manager will now optionally
use a context variable for warning filters. This is enabled by setting
the :data:`~sys.flags.context_aware_warnings` flag, either with the ``-X``
command-line option or an environment variable. This gives predicable
warnings control when using :class:`~warnings.catch_warnings` combined with
multiple threads or asynchronous tasks. The flag defaults to true for the
free-threaded build and false for the GIL-enabled build.

(Contributed by Neil Schemenauer and Kumar Aditya in :gh:`130010`.)

Other language changes
======================
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