Description

vstinner

opened on Jul 9, 2023

Issue body actions

When PyObject_SetAttr() is called with a NULL value, it's unclear if the caller wants to remove the attribute on purpose, or if the value is NULL because of an error. Such API is error prone and should be avoided.

In 2005, this feature was already deprecated in the documentation in issue #69887 by commit 45be8d6.

I propose to now emit a DeprecationWarning and schedule the removal of this feature in Python 3.15. If too many C extensions rely on this current behavior, we can consider removing the feature later.

Linked PRs

• gh-106572: Deprecate PyObject_SetAttr(v, name, NULL) #106573
• gh-106572: Convert PyObject_DelAttr() to a function #106611

Activity

1. vstinner commented on Jul 9, 2023

   

   vstinner

   on Jul 9, 2023

   MemberAuthor

   More actions

   python-dev discussion in 2015: https://mail.python.org/archives/list/[email protected]/thread/OSKS23WDHU67NQCANPNSAPHATZ2J3W7T/
2. vstinner commented on Jul 9, 2023

   

   vstinner

   on Jul 9, 2023

   MemberAuthor

   More actions

   I wrote PR #105373 to remove the deprecation in the documentation, but @serhiy-storchaka wrote that the deprecation should stay: #105374 (comment)

   Copy of his message:

   ---

   The Gmane link no longer work, the working links to the discussion (it was split on several threads) are
   https://mail.python.org/archives/list/[email protected]/thread/OSKS23WDHU67NQCANPNSAPHATZ2J3W7T/
   https://mail.python.org/archives/list/[email protected]/thread/UJI3KJQVOY7VELTDJOBTBWM3TYDHEYGD/

   The problems with the current behavior:

   1. It looks confusing that some of *Set* functions do removing, while other raise SystemError or crash. Ideally, all such functions should either support deletion by specifying NULL, or raise SystemError. A crash is acceptable too, for performance.
   2. It is a bug magnet. It is easy to get the NULL pointer instead of a pointer to object (just forgot to check for error) and delete instead of set.

   Changing the current behavior is a long process. M.-A. Lemburg specified the required procedure. It is interesting, that if we started it in 3.6, it would already be finished.
3. vstinner commented on Jul 9, 2023

   

   vstinner

   on Jul 9, 2023

   MemberAuthor

   More actions

   I reported this error prone API as capi-workgroup/problems#47
4. vstinner commented on Jul 9, 2023

   

   vstinner

   on Jul 9, 2023

   MemberAuthor

   More actions

   We should consider deprecating similar APIs used to delete an item/attribute:

   • PyObject_SetItem(obj, key, NULL): use PyObject_DelItem(obj, key) instead
   • PyMapping_SetItemString(obj, key, NULL): use PyMapping_DelItemString(obj, key) instead
   • PySequence_SetItem(obj, index, NULL): use PySequence_DelItem(obj, index) instead
   • PySequence_SetSlice(obj, i1, i2, NULL): use PySequence_DelSlice(obj, i1, i2) instead
5. vstinner

   added 3 commits that reference this issue on Jul 10, 2023

   pythongh-106572: Deprecate PyObject_SetAttr(v, name, NULL)

   d414fec

   pythongh-106572: Deprecate PyObject_SetAttr(v, name, NULL)

   1ee5f8c

   pythongh-106572: Deprecate PyObject_SetAttr(v, name, NULL)

   83ac93f
6. serhiy-storchaka commented on Jul 10, 2023

   

   serhiy-storchaka

   on Jul 10, 2023

   Member

   More actions

   Unfortunately, adding a warning will break binary compatibility. Code compiled with older Python version which uses PyObject_DelAttr() calls PyObject_SetAttr(), because PyObject_DelAttr() was implemented as a macro.

   We should first implement PyObject_DelAttr() as a separate function, then in some distant future we decide to break binary compatibility with Python older than 3.13 and add a warning, then error, in PyObject_SetAttr().
7. vstinner

   added 7 commits that reference this issue on Jul 10, 2023

   pythongh-106572: Implement PyObject_DelAttr() as a function

   a7200a0

   pythongh-106572: Convert PyObject_DelAttr() to a function

   e85cb7c

   pythongh-106572: Convert PyObject_DelAttr() to a function

   aae498b

   gh-106572: Convert PyObject_DelAttr() to a function (#106611)

   1f2921b

   pythongh-106572: Deprecate PyObject_SetAttr(v, name, NULL)

   1f5bbc2

   pythongh-106572: Deprecate PyObject_SetAttr(v, name, NULL)

   936bd14

   pythongh-106572: Deprecate PyObject_SetAttr(v, name, NULL)

   e6956af
8. vstinner

   added

   topic-C-API

   on Jul 13, 2023
9. vstinner commented on Jul 25, 2023

   

   vstinner

   on Jul 25, 2023

   MemberAuthor

   More actions

   I close the issue: see #106573 (comment)
10. vstinner

    closed this as completedon Jul 25, 2023

Issue actions