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Document that tp_setattro and tp_setattr are used for deleting attributes #69887
Description
| BPO | 25701 |
|---|---|
| Nosy | @vadmium, @serhiy-storchaka |
| Files |
Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.
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GitHub fields:
assignee = None
closed_at = <Date 2016-11-30.11:11:14.283>
created_at = <Date 2015-11-23.06:57:21.485>
labels = ['type-crash', 'docs']
title = 'Document that tp_setattro and tp_setattr are used for deleting attributes'
updated_at = <Date 2016-11-30.11:11:14.282>
user = 'https://github.com/serhiy-storchaka'bugs.python.org fields:
activity = <Date 2016-11-30.11:11:14.282>
actor = 'martin.panter'
assignee = 'docs@python'
closed = True
closed_date = <Date 2016-11-30.11:11:14.283>
closer = 'martin.panter'
components = ['Documentation']
creation = <Date 2015-11-23.06:57:21.485>
creator = 'serhiy.storchaka'
dependencies = []
files = ['41143', '41194', '41259', '41370']
hgrepos = []
issue_num = 25701
keywords = ['patch']
message_count = 17.0
messages = ['255131', '255237', '255284', '255291', '255335', '255609', '256014', '256034', '256040', '256056', '256096', '256098', '256762', '281595', '281607', '282076', '282077']
nosy_count = 4.0
nosy_names = ['docs@python', 'python-dev', 'martin.panter', 'serhiy.storchaka']
pr_nums = []
priority = 'high'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'crash'
url = 'https://bugs.python.org/issue25701'
versions = ['Python 2.7', 'Python 3.5', 'Python 3.6']
Activity
serhiy-storchaka commentedon Nov 23, 2015
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 bpo-25691).
vadmium commentedon Nov 23, 2015
Here is a patch. The same problem happens with getset descriptors:
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.
serhiy-storchaka commentedon Nov 24, 2015
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.
vadmium commentedon Nov 24, 2015
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.
serhiy-storchaka commentedon Nov 25, 2015
Python-Dev discussion:
http://comments.gmane.org/gmane.comp.python.devel/155474
vadmium commentedon Nov 30, 2015
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.
serhiy-storchaka commentedon Dec 6, 2015
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.
vadmium commentedon Dec 6, 2015
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.
vadmium commentedon Dec 7, 2015
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.
serhiy-storchaka commentedon Dec 7, 2015
The patch LGTM. Thanks Martin.
python-dev commentedon Dec 8, 2015
New changeset 50711c80ff76 by Martin Panter in branch '3.5':
Issue bpo-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 bpo-25701: Merge set and delete documentation from 3.5
https://hg.python.org/cpython/rev/7bb7173cd97a
vadmium commentedon Dec 8, 2015
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?).
serhiy-storchaka commentedon Dec 20, 2015
In 2.7 all is the same except that there is the sq_ass_slice slot and the PySequence_SetSlice() function.
vadmium commentedon Nov 24, 2016
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?
serhiy-storchaka commentedon Nov 24, 2016
Please do this yourself. This is mainly your patch.
python-dev commentedon Nov 30, 2016
New changeset 83e3c863594c by Martin Panter in branch '2.7':
Issue bpo-25701: Document that some C APIs can both set and delete items
https://hg.python.org/cpython/rev/83e3c863594c
vadmium commentedon Nov 30, 2016
I made one minor change: element → slice