Lazy Attribute

Andriy Kornatskyy andriy.kornatskyy at live.com
Fri Nov 16 11:46:43 CET 2012


I believe it is not valid relate a lazy attribute as something `cached` since it cause confusion (e.g. delete of attribute cause cached item to be re-evaluated...), `cached` and `lazy` have completely different semantic meaning... however might overlap, as we see.

Andriy


----------------------------------------
> From: steve+comp.lang.python at pearwood.info
> Subject: Re: Lazy Attribute
> Date: Fri, 16 Nov 2012 10:29:03 +0000
> To: python-list at python.org
>
> On Thu, 15 Nov 2012 15:46:19 -0700, Ian Kelly wrote:
>
> > Although you don't go into it in the blog entry, what I like about your
> > approach of replacing the descriptor with an attribute is that, in
> > addition to being faster, it makes it easy to force the object to lazily
> > reevaluate the attribute, just by deleting it.
>
> You just lost me right there. That's a poor UI design -- it violates the
> principle of least surprise. If I delete something, it should be deleted.
> Consider your example:
>
> >>>> del p.display_name
> >>>> p.display_name
> > 'Eliza Smith'
>
> That's very surprising. I am not aware of any other name in Python where
> deleting it does not remove the name from the namespace. (It is possible
> with properties, but I haven't ever come across someone who does that.)
>
> I don't have a good solution for invaliding such lazy attributes. Ideally
> we could have a new statement:
>
> refresh obj.attr # or some other name like "invalidate"
>
> but that won't happen. Other alternatives like:
>
> obj.attr.refresh()
> refresh(obj.attr)
>
> can't work because the function will see the result of the attribute
> lookup, not the lazy attribute itself. This won't do:
>
> obj.__class__.attr.refresh()
>
> because it won't know which instance to invalidate, although this could
> work:
>
> obj.__class__.attr.refresh(obj) # but it's ugly
>
> I'm very vaguely leaning towards this as the least-worst solution to
> invalidating the cached value:
>
> refresh(obj, 'attr') # pass the instance and the name
>
>
> --
> Steven
> --
> http://mail.python.org/mailman/listinfo/python-list
 		 	   		  


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