[Python-ideas] iterable: next() and __iter__() -- and __reset()

Éric Araujo merwok at netwok.org
Thu Mar 4 13:37:56 CET 2010

Hello list

Some complementary information to Chris Rebert’s message.

> (1) I do not understand an iterable type's __iter__() method to be
 > compulsary. [...]
The iterable and the iterator protocols are two different things. Every 
iterator is iterable, but not every iterable object is an iterator (see 
<http://docs.python.org/library/collections>). There are situations 
where it makes sense to use a different class as iterator (can’t find 
examples right now, hope other people will chime in), and a lot of 
situations where it’s fine to implement both protocols on the same class.

> Actually, each time I have defined one, I had to write:
>     def __iter__(self):
>         return self
Instead of returning self in __iter__ and then defining a function in 
next, i.e. returning values and raising StopIteration, you can define 
__iter__ as a generator (i.e. yield values instead of returning them) 
and not need to write next. Helpful documentation here:
<http://docs.python.org/reference/datamodel#object.__iter__> and

> (3) What I miss actually for iterables (which are their own iterator)
 > is a kind of __reset__(). In some cases, it is only needed to allow a
 > new iteration from start.[...]
Didn’t really understand your use case here, but perhaps the extension 
of the yield statement done in version 2.5 can help you here:

Hope this helps.


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