[Python-ideas] Extremely weird itertools.permutations

[Neil Girdhar]

Yes, you're right and I understand what's been done although like the 30
upvoters to the linked stackoverflow question, I find the current behaviour
surprising and would like to see a distinct_permutations function.  How do
I start to submit a patch?

Neil


On Sat, Oct 12, 2013 at 11:07 AM, Nick Coghlan <ncoghlan at gmail.com> wrote:

>
> On 12 Oct 2013 17:18, "Neil Girdhar" <mistersheik at gmail.com> wrote:
> >
> > I'm sorry, but I can't find a reference supporting the statement that
> the current permutations function is consistent with the mathematical
> definition.  Perhaps you would like to find a reference? A quick search
> yielded the book "the Combinatorics of Permutations":
> http://books.google.ca/books?id=Op-nF-mBR7YC&lpg=PP1   Please look in the
> chapter "Permutation of multisets".
>
> Itertools effectively produces the permutation of (index, value) pairs.
> Hence Steven's point about the permutations of a list not being
> mathematically defined, so you have to decide what set to map it to in
> order to decide what counts as a unique value. The mapping itertools uses
> considers position in the iterable relevant so exchanging two values that
> are themselves equivalent is still considered a distinct permutation since
> their original position is taken into account. Like a lot of mathematics,
> it's a matter of paying close attention to which entities are actually
> being manipulated and how the equivalence classes are being defined :)
>
> Hence the current proposal amounts to adding another variant that provides
> the permutations of an unordered multiset instead of those of a set of
> (index, value) 2-tuples (with the indices stripped from the results).
>
> One interesting point is that combining collections.Counter.elements()
> with itertools.permutations() currently does the wrong thing, since
> itertools.permutations() *always* considers iterable order significant,
> while for collections.Counter.elements() it's explicitly arbitrary.
>
> Cheers,
> Nick.
>
> >
> > Best,
> >
> > Neil
> >
> >
> > On Sat, Oct 12, 2013 at 2:34 AM, Steven D'Aprano <steve at pearwood.info>
> wrote:
> >>
> >> On Fri, Oct 11, 2013 at 10:55:06PM -0400, Neil Girdhar wrote:
> >> > I honestly think that Python should stick to the mathematical
> definition of
> >> > permutations rather than some kind of consensus of the tiny minority
> of
> >> > people here.
> >>
> >> So do I. And that is exactly what itertools.permutations already does.
> >>
> >>
> >>
> >> --
> >> Steven
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