[Python-checkins] [3.12] gh-103968: What's New: Add porting hints for PyType_From with metaclasses (GH-105698) (GH-106619)

[encukou]

https://github.com/python/cpython/commit/2eb9fe92b4180af0a9e51b1232df5a3d7d93788f
commit: 2eb9fe92b4180af0a9e51b1232df5a3d7d93788f
branch: 3.12
author: Miss Islington (bot) <31488909+miss-islington at users.noreply.github.com>
committer: encukou <encukou at gmail.com>
date: 2023-07-11T13:53:40+02:00
summary:

[3.12] gh-103968: What's New: Add porting hints for PyType_From with metaclasses (GH-105698) (GH-106619)

gh-103968: What's New: Add porting hints for PyType_From with metaclasses (GH-105698)
(cherry picked from commit af5cf1e75136fcef967d4ebe1bc45f29e6dc1bcf)

Co-authored-by: Petr Viktorin <encukou at gmail.com>

files:
M Doc/whatsnew/3.12.rst

diff --git a/Doc/whatsnew/3.12.rst b/Doc/whatsnew/3.12.rst
index ef00f097b8f0e..d6d7b7dac9a31 100644
--- a/Doc/whatsnew/3.12.rst
+++ b/Doc/whatsnew/3.12.rst
@@ -1836,7 +1836,31 @@ Porting to Python 3.12
   allowing incomplete initialization.
 
   Note that :c:func:`PyType_FromMetaclass` (added in Python 3.12)
-  already disallows creating classes whose metaclass overrides ``tp_new``.
+  already disallows creating classes whose metaclass overrides ``tp_new``
+  (:meth:`~object.__new__` in Python).
+
+  Since ``tp_new`` overrides almost everything ``PyType_From*`` functions do,
+  the two are incompatible with each other.
+  The existing behavior -- ignoring the metaclass for several steps
+  of type creation -- is unsafe in general, since (meta)classes assume that
+  ``tp_new`` was called.
+  There is no simple general workaround. One of the following may work for you:
+
+  - If you control the metaclass, avoid using ``tp_new`` in it:
+
+    - If initialization can be skipped, it can be done in
+      :c:member:`~PyTypeObject.tp_init` instead.
+    - If the metaclass doesn't need to be instantiated from Python,
+      set its ``tp_new`` to ``NULL`` using
+      the :const:`Py_TPFLAGS_DISALLOW_INSTANTIATION` flag.
+      This makes it acceptable for ``PyType_From*`` functions.
+
+  - Avoid ``PyType_From*`` functions: if you don't need C-specific features
+    (slots or setting the instance size), create types by :ref:`calling <call>`
+    the metaclass.
+
+  - If you *know* the ``tp_new`` can be skipped safely, filter the deprecation
+    warning out using :func:`warnings.catch_warnings` from Python.
 
 * :c:var:`PyOS_InputHook` and :c:var:`PyOS_ReadlineFunctionPointer` are no
   longer called in :ref:`subinterpreters <sub-interpreter-support>`. This is