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Date: Fri, 25 Jan 2013 15:38:16 -0800
From: Ethan Furman <ethan@stoneleaf.us>
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Subject: Re: Retrieving an object from a set
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On 01/25/2013 03:14 PM, Arnaud Delobelle wrote:
> I've got a seemingly simple problem, but for which I cannot find a
> simple solution.
>
> I have a set of objects (say S) containing an object which is equal to
> a given object (say x). So
>
>      x in S
>
> is true.  So there is an object y in S which is equal to x.  My
> problem is how to retrieve y, without going through the whole set.
> Here is a simple illustration with tuples (my actual scenario is not
> with tuples but with a custom class):
>
>>>> y = (1, 2, 3) # This is the 'hidden object'
>>>> S = set([y] + range(10000))
>>>> x = (1, 2, 3)
>>>> x in S
> True
>>>> x is y
> False
>
> I haven't found y.  It's a very simple problem, and this is the
> simplest solution I can think of:
>
> class FindEqual(object):
>      def __init__(self, obj):
>          self.obj = obj
>      def __hash__(self):
>          return hash(self.obj)
>      def __eq__(self, other):
>          equal = self.obj == other
>          if equal:
>              self.lastequal = other
>          return equal
>
>>>> yfinder = FindEqual(x)
>>>> yfinder in S
> True
>>>> yfinder.lastequal is y
> True
>
> I've found y!  I'm not happy with this as it really is a trick.  Is
> there a cleaner solution?

I don't know if there is a cleaner solution, and I quite like yours.

Can you tell us, though, why you have to have y if x == y?  Is there 
some subtle difference between the two equal objects?

~Ethan~