While generally I agree single-letter names are bad, I think `i` is pretty generally recognized as a loop counter.  Also, in astropy/,
    $ grep -r "for i in " . | wc -l
    441
    $ grep -r "for i, " . | wc -l
    130
    $ grep -r "for idx in " | wc -l
    118
    $ grep -r "for idx, " | wc -l
    69
So `i` seems to be used more often than idx, at least in this case.  I would lean toward using `i`, but I'm not some loop-counter zealot, it doesn't matter too much to me.