On Sat, Jul 6, 2013 at 7:23 PM, Steven D'Aprano <steve@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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