per instance descriptors
Simon Bunker
simon at renderIHATESPAMmania.com
Thu Dec 7 18:25:14 EST 2006
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
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