per instance descriptors

[simon at rendermania.com]

George Sakkis wrote:
> Simon Bunker wrote:
>
> > Hi I have code similar to this:
> >
> > class Input(object):
> >
> >      def __init__(self, val):
> >          self.value = val
> >
> >      def __get__(self, obj, objtype):
> >          return self.value
> >
> >      def __set__(self, obj, val):
> >          # do some checking... only accept floats etc
> >          self.value = val
> >
> > class Node(object):
> >
> > 	a = Input(1)
> > 	b = Input(2)
> >
> > I realise that a and b are now class attributes - however I want to do this:
> >
> > node1 = Node()
> > node2 = Node()
> >
> > node1.a = 3
> > node.b = 4
> >
> > And have them keep these values per instance. However now node1.a is 4
> > when it should be 3.
> >
> > Basically I want to have the Input class as a gateway that does lots of
> > checking when the attibute is assigned or read.
> >
> > I have had a look at __getattribute__(), but this gets very ugly as I
> > have to check if the attribute is an Input class or not.
> >
> > Also I don't think property() is appropriate is it? All of the
> > attributes will essentially be doing the same thing - they should not
> > have individual set/get commands.
> >
> > Is there any way of doing this nicely in Python?
>
> What about __setattr__ ? At least from your example, checking happens
> only when you set an attribute. If not, post a more representative
> sample of what you're trying to do.
>
> George

Yes, but I am setting it in the Node class aren't I? Wouldn't I need to
define __setattr__() in class Node rather than class Input? I don't
want to do this. Or am I getting confused here?

thanks

Simon