Retrieving an object from a set

[Ian Kelly]

On Fri, Jan 25, 2013 at 4:30 PM, Ian Kelly <ian.g.kelly at gmail.com> wrote:
> On Fri, Jan 25, 2013 at 4:14 PM, Arnaud Delobelle <arnodel at gmail.com> 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.
>
> You could use a dict.
>
>>>> y = (1, 2, 3)
>>>> S = {x: x for x in [y] + range(10000)}
>>>> x = (1, 2, 3)
>>>> x in S
> True
>>>> x is y
> False
>>>> S[x] is y
> True

Or you could use a set intersection:

>>> S = set([y] + list(range(10000)))
>>> S.intersection([x]).pop()
(1, 2, 3)

In my testing, the time needed for this is small and does not seem to
depend on the size of the set.