Re: 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~
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