[Python-Dev] Retrieve an arbitrary element from a set without removing it (original) (raw)

Alexander Belopolsky alexander.belopolsky at gmail.com
Sat Oct 24 19:34:48 CEST 2009

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.

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