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Document that tp_setattro and tp_setattr are used for deleting attributes #69887

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Document that tp_setattro and tp_setattr are used for deleting attributes#69887
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docsDocumentation in the Doc dirtype-crashA hard crash of the interpreter, possibly with a core dump

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@serhiy-storchaka
serhiy-storchaka
opened on Nov 23, 2015
BPO 25701
Nosy @vadmium, @serhiy-storchaka
Files
  • setattr.patch: For Python 3
  • setattr.2.patch: Add setitem slots
  • setattr.3.patch
  • setattr-2.7.patch
    		•

    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

    1. serhiy-storchaka
      assigned
      docspython
      on Nov 23, 2015
    2. serhiy-storchaka
      added
      docsDocumentation in the Doc dir
      type-crashA hard crash of the interpreter, possibly with a core dump
      on Nov 23, 2015

    3. serhiy-storchaka commented on Nov 23, 2015

      @serhiy-storchaka
      serhiy-storchaka
      on Nov 23, 2015
      MemberAuthor

      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).

    4. vadmium commented on Nov 23, 2015

      @vadmium
      vadmium
      on Nov 23, 2015
      Member

      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.

    5. serhiy-storchaka commented on Nov 24, 2015

      @serhiy-storchaka
      serhiy-storchaka
      on Nov 24, 2015
      MemberAuthor

      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.

    6. vadmium commented on Nov 24, 2015

      @vadmium
      vadmium
      on Nov 24, 2015
      Member

      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.

    7. serhiy-storchaka commented on Nov 25, 2015

      @serhiy-storchaka
      serhiy-storchaka
      on Nov 25, 2015
      MemberAuthor

      Python-Dev discussion:

      http://comments.gmane.org/gmane.comp.python.devel/155474

    8. vadmium commented on Nov 30, 2015

      @vadmium
      vadmium
      on Nov 30, 2015
      Member

      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.

    9. serhiy-storchaka commented on Dec 6, 2015

      @serhiy-storchaka
      serhiy-storchaka
      on Dec 6, 2015
      MemberAuthor

      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.

    10. vadmium commented on Dec 6, 2015

      @vadmium
      vadmium
      on Dec 6, 2015
      Member

      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.

    11. vadmium commented on Dec 7, 2015

      @vadmium
      vadmium
      on Dec 7, 2015
      Member

      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.

    12. serhiy-storchaka commented on Dec 7, 2015

      @serhiy-storchaka
      serhiy-storchaka
      on Dec 7, 2015
      MemberAuthor

      The patch LGTM. Thanks Martin.

    13. python-dev commented on Dec 8, 2015

      @python-dev
      python-devmannequin
      on Dec 8, 2015
      Mannequin

      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

    14. vadmium commented on Dec 8, 2015

      @vadmium
      vadmium
      on Dec 8, 2015
      Member

      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?).

    15. serhiy-storchaka commented on Dec 20, 2015

      @serhiy-storchaka
      serhiy-storchaka
      on Dec 20, 2015
      MemberAuthor

      In 2.7 all is the same except that there is the sq_ass_slice slot and the PySequence_SetSlice() function.

    16. vadmium commented on Nov 24, 2016

      @vadmium
      vadmium
      on Nov 24, 2016
      Member

      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?

    17. serhiy-storchaka commented on Nov 24, 2016

      @serhiy-storchaka
      serhiy-storchaka
      on Nov 24, 2016
      MemberAuthor

      Please do this yourself. This is mainly your patch.

    18. python-dev commented on Nov 30, 2016

      @python-dev
      python-devmannequin
      on Nov 30, 2016
      Mannequin

      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

    19. vadmium commented on Nov 30, 2016

      @vadmium
      vadmium
      on Nov 30, 2016
      Member

      I made one minor change: element → slice

    20. vadmium
      closed this as completedon Nov 30, 2016
    21. ezio-melotti
      transferred this issue fromon Apr 10, 2022
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