[Python-ideas] Reversable

[cool-RR]

On Tue, Apr 27, 2010 at 12:12 AM, George Sakkis <george.sakkis at gmail.com>wrote:

> On Mon, Apr 26, 2010 at 11:41 PM, cool-RR <cool-rr at cool-rr.com> wrote:
> > On Mon, Apr 26, 2010 at 11:34 PM, George Sakkis <george.sakkis at gmail.com
> >
> > wrote:
> >>
> >> On Mon, Apr 26, 2010 at 11:18 PM, cool-RR <cool-rr at cool-rr.com> wrote:
> >> > On Mon, Apr 26, 2010 at 11:13 PM, Xavier Ho <contact at xavierho.com>
> >> > wrote:
> >> >>
> >> >> On Tue, Apr 27, 2010 at 7:10 AM, cool-RR <cool-rr at cool-rr.com>
> wrote:
> >> >>>
> >> >>> Just something small that I thought of, and I haven't thought about
> >> >>> this
> >> >>> deeply at all, so maybe this is way wrong. But: What about adding a
> >> >>> `Reversable` next to all the `Iterable` and `Container` and stuff?
> >> >>
> >> >> You mean there is something Iterable we can't reverse by doing [::-1]
> >> >> or
> >> >> calling reversed() ? This idea feels a bit too general to be useful.
> >> >> Any
> >> >> rationales?
> >> >>
> >> >> Cheers,
> >> >> Xav
> >> >
> >> > As far as I know, iterables are generally not reversable. Try defining
> a
> >> > simple iterator, like a class with just an `__iter__` function, and
> run
> >> > `reversed` on it. You get `TypeError: argument to reversed() must be a
> >> > sequence`. (Which by the way is a bad error message.)
> >> > Am I missing something?
> >>
> >> So what should reversed() (or a new Reversable()) return for, say,
> >> itertools.count() ?
> >>
> >> By the way, comp.lang.python [1] or the tutor mailing list [2] are
> >> more appropriate than python-ideas for asking
> >> questions "you haven't thought about deeply at all".
> >>
> >> George
> >>
> >> [1] http://mail.python.org/mailman/listinfo/python-list
> >> [2] http://mail.python.org/mailman/listinfo/tutor
> >
> > I'm not really understanding you, George. Am I getting something very
> wrong
> > here? When you put `itertools.count()` into `reversed()`, you get an
> error,
> > like you should, because it's not a sequence and it doesn't define
> > `__reversed__`. So it's not a reversable object. I'm proposing to have a
> > `Reversable` similar to `Iterable`, which checks the existence of
> > `__reversed__` instead of `__iter__`.
>
> Sorry, it wasn't obvious (to me at least) that you were talking about
> the abstract classes under the collections module, and in your second
> post you implied (again, that was my understanding at any rate) that
> defining a class with just an __iter__ should be acceptable by
> `reversed` instead of raising a TypeError.
>
> On the other hand a collections.Reversable abstract class that checks
> specifically for __reversed__ sounds quite reasonable.
>
> George
>

Does anyone else care to express their opinion about the Reversable
suggestion?

Ram.
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