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Follow-up of bpo-36381: In Python 3.8, PyArg_ParseTuple() and Py_BuildValue() formats using "int" when PY_SSIZE_T_CLEAN is not defined, but Py_ssize_t when PY_SSIZE_T_CLEAN is defined, were deprecated by:
commit d3c72a223a5f771f964fc34557c55eb5bfa0f5a0
Author: Inada Naoki <songofacandy@gmail.com>
Date: Sat Mar 23 21:04:40 2019 +0900
 bpo-36381: warn when no PY_SSIZE_T_CLEAN defined (GH-12473)
 
 We will remove int support from 3.10 or 4.0.
I propose to drop support for these formats in Python 3.10. It is a backward incompatible change on purpose, to ensure that all C extensions are compatible with objects larger than 2 GB, and that all C extensions behave the same.
I'm not sure of the effects of this issue on bpo-27499 "PY_SSIZE_T_CLEAN conflicts with Py_LIMITED_API".

New changeset d36cf5f1d20ce9f111a8fc997104785086e8eee6 by Victor Stinner in branch 'master':
bpo-40943: Replace PY_FORMAT_SIZE_T with "z" (GH-20781)
https://github.com/python/cpython/commit/d36cf5f1d20ce9f111a8fc997104785086e8eee6

I started to write a long rationale to explain why "#" variants of formats must be deprecated, before I noticed that they are already deprecated! To not lost it, here is my rationale :-)
The C code base of Python switched from int to Py_ssize_t with PEP 353. For not break the backward compatibility, C extension modules have to opt-in for Py_ssize_t by defining PY_SSIZE_T_CLEAN macro in their code base. It affects a few format PyArg_ParseTuple() and Py_BuildValue():
https://docs.python.org/dev/c-api/arg.html
Currently, the documentation says "This behavior will change in a future Python version to only support Py_ssize_t and drop int support. It is best to always define PY_SSIZE_T_CLEAN."
Continue to use int by default prevents to accept content larger than 2 GB and causes issues with limited C API: bpo-27499.
I propose to raise a DeprecationWarning on C extension modules built without the PY_SSIZE_T_CLEAN macro defined.
#warning could be used to emit a warning when building a C extension without PY_SSIZE_T_CLEAN. The problem is that the compiler output is hidden by "pip install", only few developers pay attention to such warnings, and also we should not bother C extensions which don't use PyArg_ParseTuple() and Py_BuildValue().
I prefer to raise a DeprecationWarning exception at runtime since this warning can be made an error when using -Werror: it eases detection of deprecated modules without preventing to build or use a C extension using the deprecated functions.
Two releases after the DeprecationWarning is introduced, we can consider to always raise an exception. Again, I dislike the idea of always require PY_SSIZE_T_CLEAN macro when including "Python.h" header file. I suggest to only raise an exception at runtime when the few affected functions are called.
For example, PyArg_ParseTuple(args, "y*", &buffer) always Py_buffer which uses Py_ssize_t: the behavior does not depend on PY_SSIZE_T_CLEAN.
An alternative is to even deprecate the few "#" formats which are affected by PY_SSIZE_T_CLEAN.

New changeset 37bb2895561d3e63a631f10875567b4e33b30c07 by Victor Stinner in branch 'master':
bpo-40943: PY_SSIZE_T_CLEAN required for '#' formats (GH-20784)
https://github.com/python/cpython/commit/37bb2895561d3e63a631f10875567b4e33b30c07

libxml2 is hit by this issue:
* https://gitlab.gnome.org/GNOME/libxml2/-/issues/203
* https://gitlab.gnome.org/GNOME/libxml2/-/merge_requests/87 (my proposed fix)

There is a still warning, not error.
https://github.com/python/cpython/blob/985ac016373403e8ad41f8d563c4355ffa8d49ff/Python/getargs.c#L2535-L2542 

Why PR 20784 has been merged? Was not the plan to emit a deprecation warning first?

>Why PR 20784 has been merged? Was not the plan to emit a deprecation warning first?
We had been emitted DeprecationWarning since Python 3.8. See GH-12473.
GH-20784 changed the DeprecationWarning to SystemError.

Sorry, I was confused by Victor's long msg371219.

New changeset 569ca81adf0be92be8752f6cc6492117f9ef3c0b by Miss Islington (bot) in branch '3.10':
bpo-40943: Fix skipitem() didn't raise SystemError (GH-25937)
https://github.com/python/cpython/commit/569ca81adf0be92be8752f6cc6492117f9ef3c0b

commit 4ebf4a6bfad4afcbab3baf9c0159c7767e2a64c0
Author: Inada Naoki <songofacandy@gmail.com>
Date: Fri May 7 11:56:48 2021 +0900
 bpo-40943: Fix skipitem() didn't raise SystemError (GH-25937)
 
 `convertitem()` raises `SystemError` when '#' is used without `PY_SSIZE_T_CLEAN`.
 This commit makes `skipitem()` raise it too.