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[Python-ideas] exclusively1, common, exclusively2 = set1 - set2, set1 & set2, set2

David Mertz mertz at gnosis.cx
Sun Jul 7 04:53:09 CEST 2013 
On Sat, Jul 6, 2013 at 7:23 PM, Steven D'Aprano <steve at pearwood.info> wrote:

> For the record, your suggestion was to add a convenience method:
> set1.partition(set2) => (set1 - set2, set1 & set2)
> only1, both = set1.partition(set2)
> only2 = set2 - set1
>
> instead of Paddy's suggestion (with the method name left unknown):
>
> only1, both, only2 = set1.???????(set2)
>

I dislike the appearance of creating a method on the 'set' type.  It
creates an asymmetry between the respective sets that doesn't really
express the sense of the symmetrical operation.

However, I *do* think it would be worth having a faster (i.e. C-coded) way
of doing the three-way partitioning.  Moreover, there's really no reason
that such a function couldn't operate on collections (or iterators) other
than sets, and being general seems more useful.

Therefore, I would suggest adding a C-coded function in the
STDLIB--probably in 'collections' to do this.  E.g.

  from collections import segment  # tentative name, see various
suggestions in thread
  only1, only2, intersection = segment(iter1, iter2)

In behavior, this should do the same thing as the below (just faster):

  def segment(iter1, iter2):
      set1, set2 = map(set, (iter1, iter2))
      return set1 - set2, set2 - set1, set1 & set2



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