per instance descriptors

[Simon Bunker]

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?

thanks

Simon