per instance descriptors

[George Sakkis]

simon at rendermania.com wrote:

> 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?

Yes, __setattr__ would be defined in Node and Input would go. It seems
to me that the only reason you introduced Input was to implement this
controlled attribute access, and as you see it doesn't work as you want
it to. Why not define Node.__setattr__ ?

George