Alexander Belopolsky wrote:
On Fri, Oct 23, 2009 at 6:46 PM, Steven D'Aprano <st...@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.
As I explained in response to Steven, set()s *implicitly* map of
abstract, non-object equivalence classes to a concrete, representative
object/member of that class.
> In this model, a
get() method, or even a __getitem__ with s[k] is k, is only natural.
No, if key and value were the same thing, the get method would be
nonesense, as Guido said. But since they are not, since the implict key
is an abstract class, retrieving the representative, perhaps canonical
object given *any* member *is* natural. Being able to do so is also a
standard practice in mathematics.
Terry Jan Reedy
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