[Tutor] @property for old style classes vs new style classes

[monikajg at netzero.net]

For both old and new classes I have var only in GetSet class, but not in instance me. Why?
print me.__dict__
print GetSet.__dict__

{'attrval': 5}

{'__weakref__': <attribute '__weakref__' of 'GetSet' objects>, '__doc__': None, '__module__': '__main__', '__init__': <function GetSet.__init__ at 0x03763C90>, '__dict__': <attribute '__dict__' of 'GetSet' objects>, 'var': <property object at 0x0375E7E0>}
>>> 

Thank you very much
Monika

---------- Original Message ----------
From: Steven D'Aprano <steve at pearwood.info>
To: tutor at python.org
Subject: Re: [Tutor] @property for old style classes vs new style classes
Date: Fri, 16 Sep 2016 02:57:12 +1000

On Thu, Sep 15, 2016 at 04:40:22AM +0000, monikajg at netzero.net wrote:

> Could somebody please explain what is going on for old style classes for the below code:

The important part is that the descriptor protocol doesn't get used for 
old style classes. If that statement means something to you, that's 
great, otherwise please ask. So in an old-style class:


> class GetSet():
> 
>     def __init__(self, value):
>         self.attrval = value
> 
>     @property
>     def var(self):
>         print "getting the var attribute"
>         return self.attrval
>     @var.setter 
>     def var(self,value):
>         print "setting the var attribute"
>         self.attrval = value

At this point, you have a class GetSet, with a class attribute:

    GetSet.var

which is a property object. Now let's make an instance:

> me = GetSet(5)
> me.var = 1000

At this point, the instance now has an instance attribute:

    me.var

which is the int 1000. So there are now TWO attributes called "me", 
living in different scopes:

    me.__dict__['var'] = 1000  # in the instance

    GetSet.__dict__['var'] = property object


Remember that attribute look-ups normally look in the instance __dict__ 
before the class __dict__.


> print me.var
> del me.var
> print me.var

The first print looks for 'var' in the instance, finds it, and prints 
1000. Then the del command looks for 'var' in the instance, finds it, 
and deletes it. Then finally the second print looks for 'var' in the 
instance, *doesn't* find it there (because it has been deleted), so it 
looks in the class, and finds GetSet.__dict__['var'] which is a property 
object.

At this point, things get a bit mysterious. According to the 
documentation, Python ought to print something like:

    <property object at 0x123456>

since the descriptor protocol doesn't run for old-style classes. But 
apparently it does *partly* run, because the property object's __get__ 
method is invoked, which calls the getter method that you defined.


> Output:
> 1000
> getting the var attribute
> 5


So there is a part mystery here. As far as I can tell, the documentation 
suggests that the output should be:

1000
<property object at 0x...>

rather than what you got.




-- 
Steve
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