[Python-Dev] Retrieve an arbitrary element from a set without removing it

[Alexander Belopolsky]

On Fri, Oct 23, 2009 at 6:46 PM, Steven D'Aprano <steve at pearwood.info> wrote:
> On Sat, 24 Oct 2009 06:04:12 am Terry Reedy wrote:
..
>> fwiw, I think the use case for this is sufficiently rare that it does
>> not need a separate method just for this purpose.
>
>
> And yet it keeps coming up, again and again... obviously people using
> sets in code think it has a use-case.
>
This reminds me a debate I had with Martin several years ago:

http://bugs.python.org/issue1507011

Here is the essence:

AB> I disagree with Martin. I think interning is a set
AB> operation and it is unfortunate that set API does not
AB> support it directly.

ML> I disagree with Alexander's last remark in several respects:
ML> set is indeed a container, and there is a way to get the
ML> elements back (namely, by enumerating them, or picking an
ML> arbitrary element for removal). There is no operation to check,
ML> on insertion of E, what the the prior element equal to E was
ML> (if any); there is only a check to determine whether E is in the
ML> set. The operation "give me the member equal but not identical
ML> to E" is conceptually a lookup operation; the mathematical set
ML> construct has no such operation, and the Python set models
ML> it closely. IOW, set is *not* a dict with key==value.

To me, however, a set seems to be a container that is a specialization
of a dict with values and keys being the same.   In this model, a
get() method, or even a __getitem__ with s[k] is k, is only natural.