per instance descriptors

[Simon Bunker]

George Sakkis wrote:
> 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
> 

The __setattr__ approach is what I am hoping to avoid.

Having the input class means that I just need to assign a class to an 
attribute rather than having to filter each attribute name - really 
annoying as there will be several classes similar to Node with different 
attributes, but which should be handled in the same way.

Frankly descriptors seems exactly what I want - but having them as class 
attributes makes them completely useless!

Simon