[Python-ideas] Reversable

[Masklinn]

On 28 Apr 2010, at 12:48 , cool-RR wrote:
> 
> 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.
I'm not quite sure of the point/use case for the feature, but that's about
it.