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

[Steven D'Aprano]

On Mon, May 05, 2014 at 09:07:27AM -0700, Ram Rachum wrote:
> And now that I think about it, I'd also like to give it a `__len__`, and to 
> give `itertools.product` the same treatment. What do you think? 

Consider:

p = itertools.permutations('CAT')
assert len(p) == 6

So far, that's obvious. But:

next(p)
=> returns a permutation

Now what will len(p) return? If it still returns 6, that will lead to 
bugs when people check the len, but fail to realise that some of those 
permutations have already been consumed. In the most extreme case, you 
could have:

assert len(p) == 6
list(p) == []

which is terribly surprising.


On the other hand, if len(p) returns the number of permutations 
remaining, apart from increasing the complexity of the iterator, it will 
also be surprising to those who expect the length to be the total number 
of permutations.

I would rather have a separate API, perhaps something like this:

p.number()  # returns the total number of permutations


-- 
Steven