On Fri, Oct 23, 2009 at 6:46 PM, Steven D'Aprano <steve@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.