[Python-ideas] Introduce collections.Reiterable

[Andrew Barnert]

On Sep 19, 2013, at 15:22, Neil Girdhar <mistersheik at gmail.com> wrote:

> Why not do it the way Antoine suggested, but instead of 
> 
> self.need_cloning = isinstance(it, collections.Iterator)
> 
> have
> 
> self.need_cloning = isinstance(it, collections.Reiterable)

Because we already have Iterator today, and we don't have Reiterable, and nobody has yet come up with a useful case where the latter would do the right thing and the former wouldn't.

(Also because the second one is the exact opposite of what you meant... But I assume that's a simple typo.)

> 
> Then, mark the appropriate classes as subclasses of collections.Reiterable where collections.Sequence < collections.Reiterable < collections.Iterable?
> 
> Best,
> 
> Neil
> 
> 
> On Thu, Sep 19, 2013 at 5:40 PM, Terry Reedy <tjreedy at udel.edu> wrote:
>> On 9/19/2013 8:18 AM, Steven D'Aprano wrote:
>>> On Thu, Sep 19, 2013 at 07:12:26PM +1000, Nick Coghlan wrote:
>>> 
>>>> is there any obvious case where "iterable but
>>>> not an iterator" gives the wrong answer?
>>> 
>>> I'm not sure if it counts as "obvious", but one can write an iterator
>>> that is re-iterable. A trivial example:
>>> 
>>> class Reiter:
>>>      def __init__(self):
>>>          self.i = 0
>>>      def __next__(self):
>>>          i = self.i
>>>          if i < 10:
>>>              self.i += 1
>>>              return i
>>>          self.i = 0
>> 
>> This, I agree, is bad.
>> 
>> 
>>>          raise StopIteration
>>>      def __iter__(self):
>>>          return self
>>> 
>>> 
>>> I know that according to the iterator protocol, such a re-iterator
>>> counts as "broken":
>>> 
>>> [quote]
>>> The intention of the protocol is that once an iterator’s next() method
>>> raises StopIteration, it will continue to do so on subsequent calls.
>> 
>> I would add 'unless and until iter() or another reset method is called. Once one pokes at a iterator with another mutation method, all bets are off. I would consider Reiter less broken or not at all if the reset in __next__ were removed, since then it would continue to raise until explicity reset with __iter__
>> 
>> 
>>> Implementations that do not obey this property are deemed broken. (This
>>> constraint was added in Python 2.3; in Python 2.2, various iterators are
>>> broken according to this rule.)
>>> 
>>> http://docs.python.org/2/library/stdtypes.html#iterator-types
>>> 
>>> but clearly there is a use-case for re-iterable "things", such as dict
>>> views, which can be re-iterated over. We just don't call them iterators.
>>> So maybe there should be a way to distinguish between "oops this
>>> iterator is broken" and "yes, this object can be iterated over
>>> repeatedly, it's all good".
>>> 
>>> At the moment, dict views aren't directly iterable (you can't call
>>> next() on them). But in principle they could have been designed as
>>> re-iterable iterators.
>>> 
>>> Another example might be iterators with a reset or restart method, or
>>> similar. E.g. file objects and seek(0). File objects are officially
>>> "broken" iterators, since you can seek back to the beginning of the
>>> file. I don't think that's a bad thing.
>>> 
>>> But nor am I sure that it requires a special Reiterable class so we can
>>> test for it.
>> 
>> 
>> -- 
>> Terry Jan Reedy
>> 
>> 
>> 
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