Hi Raymond,

I agree with you on the consistency point with itertools.product.  That's a great point.

However, permutations(set(population)) is not the correct way to take the permutations of a multiset.  Please take a look at how permutations are taken from a multiset from any of the papers I linked or any paper that you can find on the internet.  The number of permutations of multiset is n! /  \prod a_i! for a_i are the element counts — just like I was taught in school.

There is currently no fast way to find these permutations of a multiset and it is a common operation for solving problems.  What is needed, I think is a function multiset_permutations that accepts an iterable.

Best,

Neil


On Sat, Oct 12, 2013 at 8:44 PM, Raymond Hettinger <raymond.hettinger@gmail.com> wrote:

On Oct 12, 2013, at 11:56 AM, Neil Girdhar <mistersheik@gmail.com> wrote:

, I find the current behaviour surprising and would like to see a distinct_permutations function.
 How do I start to submit a patch?

You can submit your patch at http://bugs.python.org and assign it to me (the module designer and maintainer).

That said, the odds of it being accepted are slim.
There are many ways to write combinatoric functions
(Knuth has a whole book on the subject) and I don't
aspire to include multiple variants unless there are
strong motivating use cases.

In general, if someone wants to eliminate duplicates
from the population, they can do so easily with:

   permutations(set(population), n)

The current design solves the most common use cases
and it has some nice properties such as:
 * permutations is a subsequence of product
 * no assumptions are made about the comparability
   or orderability of members of the population
 * len(list(permutations(range(n), r))) == n! / (n-r)! 
   just like you were taught in school
 * it is fast

For more exotic needs, I think is appropriate to look
outside the standard library to more full-featured
combinatoric libraries (there are several listed at
pypi.python.org).

  
Raymond