[Python-ideas] Introduce collections.Reiterable

Terry Reedy tjreedy at udel.edu
Thu Sep 19 23:40:20 CEST 2013

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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