[Python-Dev] The iterator story

[Oren Tirosh]

On Sat, Jul 20, 2002 at 10:09:23AM -0400, Aahz wrote:
> > That's the scenario that bit me too.  For me it was a little more difficult
> > to find because it was wrapped in a few layers of chained transformations.
> > I can't tell by the last element in the chain whether the first one is 
> > re-iterable or not. 
> > 
> > My suggestion (which was rejected by Guido) was to raise an error when an
> > iterator's .next() method is called afer it raises StopIteration.  This
> > way, if I try to iterate over the result again at least I'll get and error
> > like "IteratorExhaustedError" instead something that is indistinguishable
> > from an iterator of an empty container. I hate silent errors.
> 
> I'm still not understanding how this would help.  When a chainable
> transformer gets StopIteration, it should immediately return.  What else
> do you want to do?

The tranformations are fine the way they are.  The problem is the source - if 
the source is an exhausted iterator and you ask it for a new iterator it will 
happily return itself and report StopIteration on each .next(). This behavior 
is indistringuishable from a valid iterator on an empty container.  

What I would like is for iterators to return StopIteration exactly once and 
then switch to a different exception.  This way the transformations will not 
need to care whether their upstream source is restartable or not - the 
exception will propagate through the entire chain and notify the consumer at 
the end of the chain that the source at the beginning of the chain is not 
re-iterable.

I'm not suggesting that all iterator implementers much do this - having it on
just the builtin iterators will be a great help.

Right now I am using tricks like special-casing files and checking if 
iter(x) is x.  It works but I hate it.

	Oren