Bypassing __setattr__ for changing special attributes

[George Sakkis]

I was kinda surprised that setting __class__ or __dict__ goes through
the __setattr__ mechanism, like a normal attribute:

class Foo(object):
def __setattr__(sel f, attr, value):
pass

class Bar(object):
pass

>>f = Foo()
>>f.__class__ = Bar
>>print f.__class__ is Foo

True

Is there a way (even hackish) to bypass this, or at least achieve
somehow the same goal (change f's class) ?

George

[Ziga Seilnacht]

Re: Bypassing __setattr__ for changing special attributes

George Sakkis wrote:

I was kinda surprised that setting __class__ or __dict__ goes through
the __setattr__ mechanism, like a normal attribute:
>
class Foo(object):
def __setattr__(sel f, attr, value):
pass
>
class Bar(object):
pass
>

>f = Foo()
>f.__class__ = Bar
>print f.__class__ is Foo

True
>
Is there a way (even hackish) to bypass this, or at least achieve
somehow the same goal (change f's class) ?
>
George

>>object.__seta ttr__(f, '__class__', Bar)
>>f.__class__ is Bar

True

Ziga

[bruno.desthuilliers@gmail.com]

Re: Bypassing __setattr__ for changing special attributes

On 20 fév, 05:39, "George Sakkis" <george.sak...@ gmail.comwrote:

I was kinda surprised that setting __class__ or __dict__ goes through
the __setattr__ mechanism, like a normal attribute:

But __class__ and __dict___ *are* 'normal' attributes...

[Steven D'Aprano]

Re: Bypassing __setattr__ for changing special attributes

On Mon, 19 Feb 2007 23:18:02 -0800, Ziga Seilnacht wrote:

George Sakkis wrote:

>I was kinda surprised that setting __class__ or __dict__ goes through
>the __setattr__ mechanism, like a normal attribute:
>>
>class Foo(object):
> def __setattr__(sel f, attr, value):
> pass
>>
>class Bar(object):
> pass
>>

>>f = Foo()
>>f.__class__ = Bar
>>print f.__class__ is Foo

>True
>>
>Is there a way (even hackish) to bypass this, or at least achieve
>somehow the same goal (change f's class) ?
>>
>George

>

>>>object.__set attr__(f, '__class__', Bar)
>>>f.__class_ _ is Bar

True

This version is arguably more "correct", although a tad longer to write,
and doesn't need you to hard-code the class superclass:

super(f.__class __, f).__setattr__( '__class__', Bar)


But what surprised me was that this *didn't* work:

>>f = Foo()
>>f.__dict__['__class__'] = Bar
>>f.__class__

<class '__main__.Foo'>


Unless I'm confused, it looks like instance.__clas s__ bypasses the usual
lookup mechanism (instance.__dic t__, then instance.__clas s__.__dict__) for
some reason.

>>Foo.x = 1 # stored in class __dict__
>>f.x

1

>>f.__dict__['x'] = 2 # stored in instance __dict__
>>f.x

2

>>Foo.x

1

But __class__ doesn't behave like this. Why?



--
Steven.

[George Sakkis]

Re: Bypassing __setattr__ for changing special attributes

On Feb 20, 3:54 am, "bruno.desthuil li...@gmail.com "
<bruno.desthuil li...@gmail.com wrote:

On 20 fév, 05:39, "George Sakkis" <george.sak...@ gmail.comwrote:
>

I was kinda surprised that setting __class__ or __dict__ goes through
the __setattr__ mechanism, like a normal attribute:

>
But __class__ and __dict___ *are* 'normal' attributes...

Well, let alone for the fact that two leading and trailing underscores
means 'special' in python, these attributes are not exactly what they
seem to be, i.e. a 'normal' type or dict:

>>type(object._ _dict__['__class__'])

<type 'getset_descrip tor'>

>>type(type.__d ict__['__dict__'])

<type 'getset_descrip tor'>


George

[George Sakkis]

Re: Bypassing __setattr__ for changing special attributes

On Feb 20, 7:57 am, Steven D'Aprano
<s...@REMOVE.TH IS.cybersource. com.auwrote:

On Mon, 19 Feb 2007 23:18:02 -0800, Ziga Seilnacht wrote:

George Sakkis wrote:

I was kinda surprised that setting __class__ or __dict__ goes through
the __setattr__ mechanism, like a normal attribute:

>

class Foo(object):
def __setattr__(sel f, attr, value):
pass

>

class Bar(object):
pass

>

>f = Foo()
>f.__class__ = Bar
>print f.__class__ is Foo
True

>

Is there a way (even hackish) to bypass this, or at least achieve
somehow the same goal (change f's class) ?

>

George

>

>>object.__seta ttr__(f, '__class__', Bar)
>>f.__class__ is Bar

True

>
This version is arguably more "correct", although a tad longer to write,
and doesn't need you to hard-code the class superclass:
>
super(f.__class __, f).__setattr__( '__class__', Bar)
>
But what surprised me was that this *didn't* work:
>

>f = Foo()
>f.__dict__['__class__'] = Bar
>f.__class__

>
<class '__main__.Foo'>
>
Unless I'm confused, it looks like instance.__clas s__ bypasses the usual
lookup mechanism (instance.__dic t__, then instance.__clas s__.__dict__) for
some reason.
>

>Foo.x = 1 # stored in class __dict__
>f.x

1

>f.__dict__['x'] = 2 # stored in instance __dict__
>f.x

2

>Foo.x

>
1
>
But __class__ doesn't behave like this. Why?
>
--
Steven.

Perhaps because __class__ is a special descriptor:

>>type(object._ _dict__['__class__'])

<type 'getset_descrip tor'>

George

[Fuzzyman]

Re: Bypassing __setattr__ for changing special attributes

On Feb 20, 12:57 pm, Steven D'Aprano
<s...@REMOVE.TH IS.cybersource. com.auwrote:

On Mon, 19 Feb 2007 23:18:02 -0800, Ziga Seilnacht wrote:

George Sakkis wrote:

I was kinda surprised that setting __class__ or __dict__ goes through
the __setattr__ mechanism, like a normal attribute:

>

class Foo(object):
def __setattr__(sel f, attr, value):
pass

>

class Bar(object):
pass

>

>f = Foo()
>f.__class__ = Bar
>print f.__class__ is Foo
True

>

Is there a way (even hackish) to bypass this, or at least achieve
somehow the same goal (change f's class) ?

>

George

>

>>object.__seta ttr__(f, '__class__', Bar)
>>f.__class__ is Bar

True

>
This version is arguably more "correct", although a tad longer to write,
and doesn't need you to hard-code the class superclass:
>
super(f.__class __, f).__setattr__( '__class__', Bar)
>
But what surprised me was that this *didn't* work:
>

>f = Foo()
>f.__dict__['__class__'] = Bar
>f.__class__

>
<class '__main__.Foo'>
>
Unless I'm confused, it looks like instance.__clas s__ bypasses the usual
lookup mechanism (instance.__dic t__, then instance.__clas s__.__dict__) for
some reason.
>

>Foo.x = 1 # stored in class __dict__
>f.x

1

>f.__dict__['x'] = 2 # stored in instance __dict__
>f.x

2

>Foo.x

>
1
>
But __class__ doesn't behave like this. Why?
>

Magic attributes like __class__ are looked up on the class rather than
the instance.

Fuzzyman
http://www.voidspace.org.uk/python/articles.shtml

--
Steven.

[Facundo Batista]

Re: Bypassing __setattr__ for changing special attributes

Ziga Seilnacht wrote:

>>>object.__set attr__(f, '__class__', Bar)
>>>f.__class_ _ is Bar

True

Interesting, but... why I must do this? And, I must *always* do this?

With Foo and Bar like the OP coded (just two new style classes, f is
instance of Foo), see this:

>>f

<__main__.Foo object at 0xb7d1280c>

>>setattr(f, '__class__', Bar)
>>f

<__main__.Foo object at 0xb7d1280c>


Ok, didn't work, try your way:

>>object.__seta ttr__(f, '__class__', Bar)
>>f

<__main__.Bar object at 0xb7d1280c>


Wow! Ok, but getting back to Foo, with the *former* method:

>>setattr(f, '__class__', Foo)
>>f

<__main__.Foo object at 0xb7d1280c>


I can't explain this to myself, :(

Regards,

--
.. Facundo
..
Blog: http://www.taniquetil.com.ar/plog/
PyAr: http://www.python.org/ar/