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

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

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4 changes: 4 additions & 0 deletions Doc/deprecations/c-api-pending-removal-in-3.17.rst
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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...


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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@picnixz picnixz Nov 9, 2025

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I think this is the most portable way of indicating the warning but I'm not entirely sure (it's only used in the getter function some lines below)

#else
static PyHash_FuncDef PyHash_Func;
#endif
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