Lazy Attribute

Steven D'Aprano steve+comp.lang.python at
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
>>>> 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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