[issue25701] Document that tp_setattro and tp_setattr are used for deleting attributes
New submission from Serhiy Storchaka: tp_setattro and tp_setattr fields of PyTypeObject are used for deleting attributes. In this case the third argument is NULL. This should be documented. Not awareness of this can cause a segmentation fail (for example see issue25691). ---------- _______________________________________ Python tracker <report@bugs.python.org> <http://bugs.python.org/issue25701> _______________________________________
Changes by Serhiy Storchaka <storchaka@gmail.com>: ---------- stage: -> needs patch _______________________________________ Python tracker <report@bugs.python.org> <http://bugs.python.org/issue25701> _______________________________________
Martin Panter added the comment: Here is a patch. The same problem happens with getset descriptors:
del sys.stdout._CHUNK_SIZE SystemError: null argument to internal routine
So I also changed the documentation for “setter” descriptor functions and the tp_descr_set() method. I only looked at the Python 3 implementation, and I understand Python 2 objects work differently in some cases, so I’m not sure if my changes are apropriate for Python 2. ---------- keywords: +patch nosy: +martin.panter stage: needs patch -> patch review Added file: http://bugs.python.org/file41143/setattr.patch _______________________________________ Python tracker <report@bugs.python.org> <http://bugs.python.org/issue25701> _______________________________________
Serhiy Storchaka added the comment: Thank you Martin. In general the patch LGTM, but I'm not expert in English writing. There are other functions and slots with twofold purpose: PyObject_SetItem, PySequence_SetItem, PySequence_SetSlice, PyMapping_SetItemString, PySequenceMethods.sq_ass_item, PyMappingMethods.mp_ass_subscript. May be address them in this issue? I'm not sure about documenting the deletion for PyObject_SetAttr and like. All these functions have Del-counterpart. Deleting by Set-function with NULL may be a side effect and be deprecated in future. May be worth to discuss this on Python-Dev. For PySys_SetObject the deletion if value is NULL already is documented, but there is no PySys_DelObject. ---------- _______________________________________ Python tracker <report@bugs.python.org> <http://bugs.python.org/issue25701> _______________________________________
Martin Panter added the comment: I agree it might be safer not to document that PyObject_SetAttr etc can delete (move that change to the tp_setattro etc method slot). I’ll also review the other functions you mentioned when I get a chance. ---------- _______________________________________ Python tracker <report@bugs.python.org> <http://bugs.python.org/issue25701> _______________________________________
Serhiy Storchaka added the comment: Python-Dev discussion: http://comments.gmane.org/gmane.comp.python.devel/155474 ---------- _______________________________________ Python tracker <report@bugs.python.org> <http://bugs.python.org/issue25701> _______________________________________
Martin Panter added the comment: In the python-dev thread, Nick Coghlan gave some arguments and examples where PyObject_SetAttr() is intended to be used for deletion. So I will keep my changes for PyObject_SetAttr(), and also _SetAttrString() for consistency. My second patch documents deletion with sq_ass_item(), mp_ass_subscript(), and PySequence_SetItem(). PyObject_SetItem() does not look like it deletes items. It explicitly considers value == NULL to be an error. As a consequence, PyMapping_SetItemString() does not delete either. PySequence_SetItem() does accept NULL to delete the item. I think this is reasonable, to keep it consistent with sq_ass_item(), so I documented it. PySequence_SetSlice() also accepts NULL to delete the slice. But I am more reluctant to document this, because I don’t know of any slot or other code that would benefit. So I have left it as is for the time being. ---------- Added file: http://bugs.python.org/file41194/setattr.2.patch _______________________________________ Python tracker <report@bugs.python.org> <http://bugs.python.org/issue25701> _______________________________________
Serhiy Storchaka added the comment: I think the conclusion of Python_Dev discussion is that it is not worth to deprecate this behavior in the code. Current behavior of PyObject_SetAttr and PySequence_SetItem should be documented with a deprecation notice. ---------- _______________________________________ Python tracker <report@bugs.python.org> <http://bugs.python.org/issue25701> _______________________________________
Martin Panter added the comment: Okay, I will look at making it say deleting via SetAttr etc is possible but is not the main purpose, and using DelAttr etc is recommended instead. ---------- _______________________________________ Python tracker <report@bugs.python.org> <http://bugs.python.org/issue25701> _______________________________________
Martin Panter added the comment: setattr.3.patch changes to documenting setting as the main purpose, but mentions deleting also works but is deprecated in favour of the main deletion functions. I also clarified that the slots have to support deleting via NULL. ---------- Added file: http://bugs.python.org/file41259/setattr.3.patch _______________________________________ Python tracker <report@bugs.python.org> <http://bugs.python.org/issue25701> _______________________________________
Serhiy Storchaka added the comment: The patch LGTM. Thanks Martin. ---------- _______________________________________ Python tracker <report@bugs.python.org> <http://bugs.python.org/issue25701> _______________________________________
Roundup Robot added the comment: New changeset 50711c80ff76 by Martin Panter in branch '3.5': Issue #25701: Document C API functions that both set and delete objects https://hg.python.org/cpython/rev/50711c80ff76 New changeset 7bb7173cd97a by Martin Panter in branch 'default': Issue #25701: Merge set and delete documentation from 3.5 https://hg.python.org/cpython/rev/7bb7173cd97a ---------- nosy: +python-dev _______________________________________ Python tracker <report@bugs.python.org> <http://bugs.python.org/issue25701> _______________________________________
Martin Panter added the comment: I committed my patch to 3.5+. I will leave this open in case someone wants to look into how things work in 2.7. I presume things might be different in Python 2; all I know is there are two kinds of classes and objects, and the slots are a bit different (__setslice__?). ---------- versions: -Python 3.4 _______________________________________ Python tracker <report@bugs.python.org> <http://bugs.python.org/issue25701> _______________________________________
Serhiy Storchaka added the comment: In 2.7 all is the same except that there is the sq_ass_slice slot and the PySequence_SetSlice() function. ---------- Added file: http://bugs.python.org/file41370/setattr-2.7.patch _______________________________________ Python tracker <report@bugs.python.org> <http://bugs.python.org/issue25701> _______________________________________
Martin Panter added the comment: The 2.7 patch looks okay to me. I confirmed that sq_ass_slice gets called to delete slices, and I presume the other stuff is similar to Python 3. Do you want to commit this to 2.7 Serhiy? ---------- stage: patch review -> commit review _______________________________________ Python tracker <report@bugs.python.org> <http://bugs.python.org/issue25701> _______________________________________
Serhiy Storchaka added the comment: Please do this yourself. This is mainly your patch. ---------- _______________________________________ Python tracker <report@bugs.python.org> <http://bugs.python.org/issue25701> _______________________________________
Roundup Robot added the comment: New changeset 83e3c863594c by Martin Panter in branch '2.7': Issue #25701: Document that some C APIs can both set and delete items https://hg.python.org/cpython/rev/83e3c863594c ---------- _______________________________________ Python tracker <report@bugs.python.org> <http://bugs.python.org/issue25701> _______________________________________
Martin Panter added the comment: I made one minor change: element → slice ---------- resolution: -> fixed stage: commit review -> resolved status: open -> closed _______________________________________ Python tracker <report@bugs.python.org> <http://bugs.python.org/issue25701> _______________________________________
participants (3)
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Martin Panter -
Roundup Robot
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Serhiy Storchaka