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Date: Fri, 25 Jan 2013 23:14:41 +0000
Subject: Retrieving an object from a set
From: Arnaud Delobelle <arnodel@gmail.com>
To: Python <python-list@python.org>
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Dear Pythoneers,

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?

-- 
Arnaud