Inconsistent behavior of descriptors

**John Roth** · Jul 18 '05, 03:06 AM

Re: Inconsistent behavior of descriptors

"Denis S. Otkidach" <ods@strana.r u> wrote in message
news:mailman.10 64938610.30070. python-list@python.org ...[color=blue]
>
> I've noticed that the order of attribute lookup is inconsistent
> when descriptor is used. property instance takes precedence of
> instance attributes:
>[color=green][color=darkred]
> >>> class A(object):[/color][/color]
> ... def _get_attr(self) :
> ... return self._attr
> ... attr = property(_get_a ttr)
> ...[color=green][color=darkred]
> >>> a=A()
> >>> a.__dict__[/color][/color]
> {}[color=green][color=darkred]
> >>> a.__dict__['attr']=1
> >>> a.__dict__[/color][/color]
> {'attr': 1}[color=green][color=darkred]
> >>> a.attr[/color][/color]
> Traceback (most recent call last):
> File "<stdin>", line 1, in ?
> File "<stdin>", line 3, in _get_attr
> AttributeError: 'A' object has no attribute '_attr'
>
> But it doesn't when I use custom class of descriptor:
>[color=green][color=darkred]
> >>> class descr(object):[/color][/color]
> ... def __get__(self, inst, cls):
> ... return inst._attr
> ...[color=green][color=darkred]
> >>> class B(object):[/color][/color]
> ... attr = descr()
> ...[color=green][color=darkred]
> >>> b=B()
> >>> b.__dict__[/color][/color]
> {}[color=green][color=darkred]
> >>> b.__dict__['attr']=1
> >>> b.__dict__[/color][/color]
> {'attr': 1}[color=green][color=darkred]
> >>> b.attr[/color][/color]
> 1
>
> Subclasses of property behave like property itself:
>[color=green][color=darkred]
> >>> class descr2(property ):[/color][/color]
> ... def __get__(self, inst, cls):
> ... return inst._attr
> ...[color=green][color=darkred]
> >>> class C(object):[/color][/color]
> ... attr = descr2()
> ...[color=green][color=darkred]
> >>> c=C()
> >>> c.__dict__[/color][/color]
> {}[color=green][color=darkred]
> >>> c.__dict__['attr']=1
> >>> c.__dict__[/color][/color]
> {'attr': 1}[color=green][color=darkred]
> >>> c.attr[/color][/color]
> Traceback (most recent call last):
> File "<stdin>", line 1, in ?
> File "<stdin>", line 3, in __get__
> AttributeError: 'C' object has no attribute '_attr'
>
> Is it an undocumented feature or I have to submit a bug report?[/color]

I'm not entirely sure what you're describing, however, I
notice that your descriptor object has a __get__ method,
but no __set__ or __del__ methods. The lookup chain
is quite different for descriptors with only __get__ methods
than it is for descriptors for all three.

In particular, it's intended that instance attributes should
be ignored if there is a data descriptor (that is, a descriptor
with both__get__ and __set__ methods.)

I'm not sure if this applies to your problem.

John Roth[color=blue]
>
> --
> Denis S. Otkidach
> http://www.python.ru/ [ru]
>
>[/color]

**Bengt Richter** · Jul 18 '05, 03:08 AM

Re: Inconsistent behavior of descriptors

On Tue, 30 Sep 2003 20:15:10 +0400 (MSD), "Denis S. Otkidach" <ods@strana.r u> wrote:
[color=blue]
>
>I've noticed that the order of attribute lookup is inconsistent
>when descriptor is used. property instance takes precedence of
>instance attributes:
>[color=green][color=darkred]
>>>> class A(object):[/color][/color]
>... def _get_attr(self) :
>... return self._attr
>... attr = property(_get_a ttr)
>...[color=green][color=darkred]
>>>> a=A()
>>>> a.__dict__[/color][/color]
>{}[color=green][color=darkred]
>>>> a.__dict__['attr']=1
>>>> a.__dict__[/color][/color]
>{'attr': 1}[color=green][color=darkred]
>>>> a.attr[/color][/color]
>Traceback (most recent call last):
> File "<stdin>", line 1, in ?
> File "<stdin>", line 3, in _get_attr
>AttributeError : 'A' object has no attribute '_attr'
>
>But it doesn't when I use custom class of descriptor:
>[color=green][color=darkred]
>>>> class descr(object):[/color][/color]
>... def __get__(self, inst, cls):
>... return inst._attr
>...[color=green][color=darkred]
>>>> class B(object):[/color][/color]
>... attr = descr()
>...[color=green][color=darkred]
>>>> b=B()
>>>> b.__dict__[/color][/color]
>{}[color=green][color=darkred]
>>>> b.__dict__['attr']=1
>>>> b.__dict__[/color][/color]
>{'attr': 1}[color=green][color=darkred]
>>>> b.attr[/color][/color]
>1
>
>Subclasses of property behave like property itself:
>[color=green][color=darkred]
>>>> class descr2(property ):[/color][/color]
>... def __get__(self, inst, cls):
>... return inst._attr
>...[color=green][color=darkred]
>>>> class C(object):[/color][/color]
>... attr = descr2()
>...[color=green][color=darkred]
>>>> c=C()
>>>> c.__dict__[/color][/color]
>{}[color=green][color=darkred]
>>>> c.__dict__['attr']=1
>>>> c.__dict__[/color][/color]
>{'attr': 1}[color=green][color=darkred]
>>>> c.attr[/color][/color]
>Traceback (most recent call last):
> File "<stdin>", line 1, in ?
> File "<stdin>", line 3, in __get__
>AttributeError : 'C' object has no attribute '_attr'
>
>Is it an undocumented feature or I have to submit a bug report?
>[/color]
I think (not having read the above carefully) that it's all documented in

404 Not Found

http://users.rcn.com/python/download/Descriptor.htm

(thanks to Raymond Hettinger).

excerpt:
"""
The details of invocation depend on whether obj is an object or a class.
Either way, descriptors only work for new style objects and classes. A
class is new style if it is a subclass of object.

For objects, the machinery is in object.__getatt ribute__ which
transforms b.x into type(b).__dict_ _['x'].__get__(b, type(b)). The
implementation works through a precedence chain that gives data
descriptors priority over instance variables, instance variables
priority over non-data descriptors, and assigns lowest priority to
__getattr__ if provided. The full C implementation can be found in
PyObject_Generi cGetAttr() in Objects/object.c.

For classes, the machinery is in type.__getattri bute__ which transforms
B.x into B.__dict__['x'].__get__(None, B). In pure Python, it looks
like:

def __getattribute_ _(self, key):
"Emulate type_getattro() in Objects/typeobject.c"
v = object.__getatt ribute__(self, key)
if hasattr(v, '__get__'):
return v.__get__(None, self)
return v

The important points to remember are:

descriptors are invoked by the __getattribute_ _ method
overriding __getattribute_ _ prevents automatic descriptor calls
__getattribute_ _ is only available with new style classes and objects
object.__getatt ribute__ and type.__getattri bute__ make different calls to __get__.
data descriptors always override instance dictionaries.
non-data descriptors may be overridden by instance dictionaries.
"""

Regards,
Bengt Richter

Inconsistent behavior of descriptors

Inconsistent behavior of descriptors

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