Title: new style object __dict__[name] ignored
Type: Severity: normal
Components: Core Versions: 2.5.1
Status: closed Resolution: fixed
Dependencies: Superseder:
Assigned To: pjenvey Nosy List: iamedu, pjenvey
Priority: Keywords:

Created on 2010-01-06.13:19:11 by iamedu, last changed 2010-04-09.03:29:41 by pjenvey.

msg5410 (view) Author: Eduardo Diaz (iamedu) Date: 2010-01-06.13:19:10
This is a bit strange, here is the sample:

class Attr(object):
    def __init__(self, name): = name

    def __set__(self, instance, value):
        instance.__dict__[] = value

class Test(object):
    attr = Attr('attr')

    def test(self):
        self.attr = 5

t = Test()
print t.attr
print t.attr

In python, if you have a newstyle object called x, and in some point define the attribute __dict__["x"] as 5 then getting the value of x. So in python that program would return:

<__main__.Attr object at 0x7f1b820b7f90>

On jython it returns:

<__main__.Attr object at 0x1>
<__main__.Attr object at 0x1>

A workaround would be to add a __get__ method something like this:

    def __get__(self, instance, owner):
        return instance.__dict__[]

But for compatibility it would be nice to change this.

msg5415 (view) Author: Philip Jenvey (pjenvey) Date: 2010-01-06.18:34:18
Thanks for logging this, I just went over it with Chris McDonough yesterday. pypy also suffers from this
msg5422 (view) Author: Philip Jenvey (pjenvey) Date: 2010-01-10.22:30:44
Jython/Pypy seem more correct than CPython according to the rules on

"For instance bindings, the precedence of descriptor invocation depends on the which descriptor methods are defined. Normally, data descriptors define both __get__() and __set__(), while non-data descriptors have just the __get__() method. Data descriptors always override a redefinition in an instance dictionary. In contrast, non-data descriptors can be overridden by instances."

"A descriptor can define any combination of __get__(), __set__() and __delete__(). If it does not define __get__(), then accessing the attribute even on an instance will return the descriptor object itself. If the descriptor defines __set__() and/or __delete__(), it is a data descriptor; if it defines neither, it is a non-data descriptor."

But I don't think we can fault CPython as it's had this behavior forever
msg5648 (view) Author: Philip Jenvey (pjenvey) Date: 2010-04-09.03:29:40
fixed in r7007

I also brought this up with the PyPy guys as their behavior matched jython's

There was a discussion on python-dev about it. The conclusion came to was that we need to match CPython and that those data descriptor docs needed to be fixed too
Date User Action Args
2010-04-09 03:29:41pjenveysetstatus: open -> closed
resolution: fixed
messages: + msg5648
2010-01-10 22:30:45pjenveysetmessages: + msg5422
2010-01-06 18:34:18pjenveysetassignee: pjenvey
messages: + msg5415
nosy: + pjenvey
2010-01-06 13:19:12iameducreate