[Python-ideas] Implement `itertools.permutations.__getitem__` and `itertools.permutations.index`

[Steven D'Aprano]

On Mon, May 05, 2014 at 06:17:16AM -0700, Ram Rachum wrote:
> I suggest implementing:
> 
>  - `itertools.permutations.__getitem__`, for getting a permutation by its 
> index number, and possibly also slicing, and 
>  - `itertools.permutations.index` for getting the index number of a given 
> permutation. 
> 
> What do you think? 

An intriguing idea.

range() objects also implement indexing, and len. But range() objects 
have an obvious, unambiguous order: range(2, 6) *must* give 2, 3, 4, 5, 
in that order, by the definition of range. Permutations aren't like 
that. The order of the permutations is an implementation detail, not 
part of the definition. If permutations provides indexing operations, 
then the order becomes part of the interface. I'm not sure that's such a 
good idea.

I think, rather that adding __getitem__ to permutations, I would rather 
see a separate function (not iterator) which returns the nth 
permutation.


-- 
Steven