Austin Bingham wrote:
On Thu, Oct 15, 2009 at 7:49 PM, Ethan Furman <et...@stoneleaf.us> wrote:
Austin Bingham wrote:
I'm feeling really dense about now... What am I missing?
What you're missing is the entire discussion up to this point. I was
looking for a way to use an alternative uniqueness criteria in a set
instance without needing to modify my class.
So is that the behavior you're wanting, keeping the first object and
discarding all others? Or is there something else I'm still missing?
Yes and yes. I want "normal" set behavior, but I want the set to use
user-provided hash and equality tests, i.e. ones that don't
necessarily call __hash__ and __eq__ on the candidate elements.
Austin
Your original post (10/15) never mentioned the __eq__() method, and
people went off on a tangent discussing hash functions. But from what
you've been saying more recently, you want a set-like collection whose
behavior isn't related to the standard comparison function.
The standard set is intimately tied to ==, in the sense that if two
objects meet the a==b relationship, then only one will be in the set.
And it doesn't matter which one, since they're equal.
You want a new collection that works differently. Great. It shouldn't
be that hard to write, but if you try to base it on existing set you'll
need a second wrapper object. If you try to base it on dict, you'll
need a second key object. You've said you don't want any extra objects
to be needed, so you'll need custom code.
A couple of dozen lines should do for the basic collection (add, clear,
remove). But if you also want difference, symmetric_difference,
intersection_update, issubset ... it could grow to several dozen. And
you'd have to make some interesting decisions, once you have two
pseudo-sets with possibly different comparison operators.
The only downside I can see is that apparently the built-in set is
written in C, and if you're implementing this in pure python, it could
be a lot slower. On the other hand, in many cases, the comparison
function-object might be the time-critical piece, in which case you coud
be close.
DaveA
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