[Python-Dev] PEP 322: Reverse Iteration

[Alex Martelli]

On Tuesday 04 November 2003 12:26 am, Guido van Rossum wrote:
> > > BTW, Alex said he was +1 on the idea, but only +0 on it being a
> > > builtin.
> >
> > Uh, did I?  OK maybe I did.  But what about "revrange" (which I'd LOVE
> > to incarnate as an iterator-returning irange with an optional reverse=
> > argument) -- was that knocked out of contention?  I claimed that just
> > revrange would be too specialized BUT irange would be JUST RIGHT...
>
> This surprised me a bit too.  The majority of Raymond's examples in
> the PEP (when I last saw it a week ago) were reverse numeric ranges,
> usually of the form revrange(n) -- which we currently have to spell as
> range(n-1, -1, -1) (I think :-) and which the new proposal would turn
> into reversed(range(n)).  According to Raymond, a built-in that would
> do just that only drew (a small number of) negative responses in the
> newsgroup.
>
> Such a thing would face zero opposition if it was part of itertools:
> itertools.revrange([start, ] stop[, step]) makes total sense to me...

And what about irange with an optional reverse= argument?  I did have
(and write about on c.l.py) a case where I currently code:

    if godown:
        iseq = xrange(len(sq)-1, start-1, -1)
    else:
        iseq = xrange(start, len(sq), 1)
    for index in iseq:
        ...

and would be just delighted to be able to code, instead,

    for index in irange(start, len(sq), reverse=godown):
        ...

Even when the need to reverse can more easily be hardwired in
the source (a more common case), would

    for index in irange(start, stop, reverse=True):

be really so much worse than

    for index in revrange(start, stop):

...?


Alex