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Wed Nov 5 11:09:14 EST 2003 · https://mail.python.org/pipermail/python-dev/2003-November/040011.html

On Wednesday 05 November 2003 03:54 pm, Phillip J. Eby wrote:
   ...
> >reverse iteration.  The iterator object has no way of knowing in advance
> >that it is going to be called by reversed().
>
> Why not change enumerate() to return an iterable, rather than an
> iterator?  Then its __reversed__ method could attempt to delegate to the
> underlying iterable.  Is it likely that anyone relies on enumerate() being
> an iterator, rather than an iterable?

I do rely on the _argument_ of enumerate being allowed to be just any 
iterator, yes -- e.g. in such idioms as:

    for i, x in enumerate(xs):
        if isgoodenough(x): return x
        elif istoohigh(i): raise GettingBoredError, i

Yes, I _could_ recode that as:

    i = 0
    for x in xs:
        if isgoodenough(x): return x
        i += 1
        if istoohigh(i): raise GettingBoredError, i

but, I don't _wanna_...:-).  enumerate is just too slick!

Of course, it would be fine for reverse(enumerate(x)) to fail for unsuitable
values of x -- that's a separate issue.

But actually it would not be a tragedy if I couldn't reverse(enumerate --
e.g. where I'd LIKE to code:

    for i, x in reverse(enumerate(xs)):
        if isbad(x): raise BadXError, x
        xs[i] = transform(x)

I _might_ reasonably code:

    for i, x in enumerate(reverse(xs)):
        if isbad(x): raise BadXError, x
        xs[-1-i] = transform(x)

that -1-i may not be the prettiest sight in the world, but I think this STILL
beats the alternative of:

    for i in reversed_range(len(xs)):
        x = xs[i]
        if isbad(x): raise BadXError, x
        xs[i] = transform(x)

not to mention today's

    for i in xrange(-1, -len(xs)-1, -1):
        x = xs[i]
        if isbad(x): raise BadXError, x
        xs[i] = transform(x)

or:

    for i in xrange(len(xs)-1, -1, -1):
        x = xs[i]
        if isbad(x): raise BadXError, x
        xs[i] = transform(x)

Alex

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Showing content from https://mail.python.org/pipermail/python-dev/2003-November/040011.html below:

[Python-Dev] Re: PEP 322: Reverse Iteration