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gh-141226: Deprecate PEP-456 support for embedders #141287

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Merged
picnixz merged 5 commits into from
Feb 21, 2026
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gh-141226: Deprecate PEP-456 support for embedders #141287

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46bd556
Deprecate PEP 456 support for embedders
picnixz Nov 9, 2025
8d1ff1c
Merge branch 'main' into feat/deprecate/external-141226
picnixz Feb 7, 2026
bc00153
defer removal to Python 3.19
picnixz Feb 7, 2026
07e32b0
fixup
picnixz Feb 7, 2026
077d808
move deprecation notice under the C API section instead
picnixz Feb 7, 2026
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4 changes: 4 additions & 0 deletions Doc/deprecations/c-api-pending-removal-in-3.17.rst
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@@ -0,0 +1,4 @@
Pending removal in Python 3.17
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

* :pep:`456` embedders support for the string hashing scheme definition.
4 changes: 4 additions & 0 deletions Doc/deprecations/index.rst
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Expand Up @@ -18,6 +18,10 @@ C API deprecations

.. include:: c-api-pending-removal-in-3.15.rst

.. include:: c-api-pending-removal-in-3.16.rst
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@hugovk hugovk Nov 9, 2025

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Good spot, we'll also need to backport this fix.

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@picnixz picnixz Nov 9, 2025

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Do you prefer that I open a separate PR for that? I can definitely do so for miss-ilington.


.. include:: c-api-pending-removal-in-3.17.rst

.. include:: c-api-pending-removal-in-3.18.rst

.. include:: c-api-pending-removal-in-3.20.rst
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12 changes: 12 additions & 0 deletions Doc/whatsnew/3.15.rst
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Expand Up @@ -844,6 +844,18 @@ Deprecated
New deprecations
----------------

* Build:

* Deprecate :pep:`456` support for providing an external definition
of the string hashing scheme. Removal is scheduled for Python 3.17.
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@hugovk hugovk Nov 9, 2025

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Any reason to rush this and not use the preferred five years?

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@picnixz picnixz Nov 9, 2025

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Err.. I forgot the 5 years. I did a +2 version but I can definitely do a +5 years. However, note that I couldn't find any (public) code that was using this feature so I guessed we could drop this.

Actually, as I told Victor, the maintenance cost on our side won't change at all because we already have all the infrastructure in place except for actually enabling this feature during the build (but all the code is already there and is straightforward). What is however hard to maintain are the docs for that (it's not easy to explain what needs to be done) and if users use this feature wrongly, it becomes a really bad footgun.

I think "advanced users" (embedders) would rather patch the entire file instead of relying on this as our configure script doesn't even support this...

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@picnixz picnixz Feb 7, 2026

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I decided to push the deprecation to 3.19. That way, if people are actually using that feature by directly passing the CFLAG and manually link their own definition, we have time to add the correct configure support instead.


Previously, embedders could define :c:macro:`Py_HASH_ALGORITHM` to be
``Py_HASH_EXTERNAL`` to indicate that the hashing scheme was provided
externally but this feature was undocumented, untested and most likely
unused.

(Contributed by Bénédikt Tran in :gh:`141226`.)

* CLI:

* Deprecate :option:`-b` and :option:`!-bb` command-line options
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3 changes: 3 additions & 0 deletions Misc/NEWS.d/next/Core_and_Builtins/2025-11-09-15-44-58.gh-issue-141226.KTb_3F.rst
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Deprecate :pep:`456` support for providing an external definition
of the string hashing scheme. Removal is scheduled for Python 3.17.
Patch by Bénédikt Tran.
2 changes: 1 addition & 1 deletion Python/pyhash.c
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Expand Up @@ -17,7 +17,7 @@
_Py_HashSecret_t _Py_HashSecret = {{0}};

#if Py_HASH_ALGORITHM == Py_HASH_EXTERNAL
extern PyHash_FuncDef PyHash_Func;
Py_DEPRECATED(3.15) extern PyHash_FuncDef PyHash_Func;
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#else
static PyHash_FuncDef PyHash_Func;
#endif
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