steve+comp.lang.python at pearwood.info
Fri Nov 16 11:29:03 CET 2012
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
> '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:
can't work because the function will see the result of the attribute
lookup, not the lazy attribute itself. This won't do:
because it won't know which instance to invalidate, although this could
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
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