classification
Title: super() and property inheritance behavior
Type: behavior Stage: needs patch
Components: Extension Modules Versions: Python 3.5
process
Status: open Resolution:
Dependencies: Superseder:
Assigned To: Nosy List: Ronny.Pfannschmidt, alex, asvetlov, cvrebert, dabeaz, daniel.urban, eric.araujo, florian-rathgeber, josmiley, ncoghlan, torsten, 猫.黒
Priority: normal Keywords: patch

Created on 2012-05-31 03:04 by 猫.黒, last changed 2014-01-31 22:28 by yselivanov.

Files
File name Uploaded Description Edit
super_setattr.patch daniel.urban, 2012-06-04 19:26 __setattr__ and __delattr__ for super review
44560_44559.diff torsten, 2012-08-22 16:00 Alternate patch, targeted at Python 2.7
Messages (8)
msg161980 - (view) Author: 猫 黒 (猫.黒) Date: 2012-05-31 03:04
super() objects allow access to inherited properties fget() but not fset() or fdel(), resulting in unexpected behavior.

Today on pydev thread 'Property inheritance in Python' GvR said "I
don't see the need for a Python-Ideas detour. It seems worth fixing"

>>> class BaseProp(object):
...     @property
...     def p(self):
...         return self._p
...     @p.setter
...     def p(self, value):
...         self._p = value
>>> class DerivedProp(BaseProp):
...     @property
...     def p(self):
...         return super(DerivedProp, self).p * 2
...     @p.setter
...     def p(self, value):
...         super(DerivedProp, self).p = value / 2
>>> d = DerivedProp()
>>> d._p = 21
>>> d.p
42
>>> d.p = 50
Traceback (most recent call last):
   ...
AttributeError: 'super' object has no attribute 'p'
msg162050 - (view) Author: josmiley (josmiley) Date: 2012-06-01 05:31
>>> class DerivedProp(BaseProp):
...     @property
...     def p(self):
...         return super(DerivedProp, self).p * 2
...     @p.setter
...     def p(self, value):
...         BaseProp.p.__set__(self,value / 2)
msg162283 - (view) Author: Daniel Urban (daniel.urban) * Date: 2012-06-04 19:26
I'm attaching a patch implementing super.__setattr__ (and __delattr__).

The implementation in the patch only works, if super can find a data descriptor in the MRO, otherwise it throws an AttributeError. As it can be seen in the tests, in some cases this may result in counter-intuitive behaviour. But I wasn't able to find another behaviour, that is consistent with both super.__getattr__ and normal __setattr__ semantics.
msg168895 - (view) Author: Torsten Landschoff (torsten) Date: 2012-08-22 16:00
I stumbled across this omission as well in 2010 and brought this up on python-dev:

http://mail.python.org/pipermail/python-dev/2010-April/099672.html

There were no replies, but perhaps my post adds a bit of information and also there is another patch linked from there. I attached my patch from 2010 for reference.
msg174404 - (view) Author: Andrew Svetlov (asvetlov) * (Python committer) Date: 2012-11-01 12:04
I'm -0 for proposed changes, these changes reduce code readability from my perspective.
I think better to use existing approach: explicitly specify what do you want to do with overloaded properties.
msg174863 - (view) Author: 猫 黒 (猫.黒) Date: 2012-11-05 05:02
I'm not a python dev, but would you say

super(self.__class__, self.__class__).x.fset(self, value)

is more readable than

super().x = value
msg174911 - (view) Author: Andrew Svetlov (asvetlov) * (Python committer) Date: 2012-11-05 14:37
I would say

@x.deleter
def x(self):
    del super().x

confuses me a bit. 
But I'm only -0, let's see other developers for their opinions.
msg179217 - (view) Author: David Beazley (dabeaz) Date: 2013-01-06 20:20
Just as a note, there is a distinct possibility that a "property" in a superclass could be some other kind of descriptor object that's not a property.  To handle that case, the solution of

super(self.__class__, self.__class__).x.fset(self, value)

would actually have to be rewritten as

super(self.__class__, self.__class__).x.__set__(self, value)

That said, I agree it would be nice to have a simplified means of accomplishing this.
History
Date User Action Args
2014-01-31 22:28:33yselivanovsetversions: + Python 3.5, - Python 3.2, Python 3.3
2013-03-21 20:23:49florian-rathgebersetnosy: + florian-rathgeber
2013-02-24 01:08:24r.david.murraylinkissue783528 superseder
2013-01-06 20:20:16dabeazsetnosy: + dabeaz
messages: + msg179217
2012-11-05 14:37:02asvetlovsetmessages: + msg174911
2012-11-05 05:02:48猫.黒setmessages: + msg174863
2012-11-01 12:04:42asvetlovsetnosy: + asvetlov
messages: + msg174404
2012-10-30 20:21:14Ronny.Pfannschmidtsetnosy: + Ronny.Pfannschmidt
2012-10-30 13:42:37r.david.murraylinkissue16363 superseder
2012-08-24 08:30:38cvrebertsetnosy: + cvrebert
2012-08-22 16:00:02torstensetfiles: + 44560_44559.diff
nosy: + torsten
messages: + msg168895

2012-06-04 19:27:15daniel.urbansetcomponents: + Extension Modules, - Library (Lib)
2012-06-04 19:26:52daniel.urbansetfiles: + super_setattr.patch

nosy: + daniel.urban
messages: + msg162283

keywords: + patch
2012-06-02 05:04:28ncoghlansetnosy: + ncoghlan
2012-06-01 05:31:41josmileysetnosy: + josmiley
messages: + msg162050
2012-06-01 02:53:22eric.araujosetnosy: + eric.araujo
stage: needs patch

versions: + Python 3.3
2012-05-31 03:07:08alexsetnosy: + alex
2012-05-31 03:04:01猫.黒create