Retrieving an object from a set

[MRAB]

On 2013-01-25 23:14, Arnaud Delobelle wrote:
> 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
>
You could first limit the search to only those which it could be:

     S & set([y])

A search would be:

 >>> f = [m for m in S & set([y]) if m is y][0]
 >>> f is y
True