how to cause a request for a missing class attribute cause its calculation

[Ethan Furman]

Ethan Furman wrote:
> Vincent Davis wrote:
>> Lets say I have class foo(object):
>>     def __init__(self, x, y):
>>         self.x=x
>>         self.y=y
>>     def xplusy(self):
>>         self.xy = x+y
>                      ^ this needs to be self.x + self.y
>>
>> inst = foo(1,2)
>> inst.xy   # no value, but I what this to cause the calculation of inst.xy
>>
>> I don't what to have self.xy calculated before it is called.
> 
> My current favorite method:
> 
>     def __getattr__(self, name):
>         if name != 'xy':
>             raise AttributeError("%s not found" % name)
>         self.xy = self.x + self.y
>         return self.xy
> 
> 
> ~Ethan~

Chris' reply is more on-point, I think -- I was thinking of attributes 
that are only calculated once.  *sigh*

~Ethan~