[Python-Dev] Single- vs. Multi-pass iterability

[Oren Tirosh]

On Fri, Jul 12, 2002 at 07:14:21AM -0400, David Abrahams wrote:
> > I'm not too thrilled about it, either, but I don't think it's too bad. If
> > you implement an object with an __iter__ method you must be aware of the
> > iteration protocol and the next method.  If you put a next method on an
> > iterable you are most probably confusing iterators and iterables and not
> > just using the name 'next' for some other innocent purpose.
> 
> People may have already written that innocent code, but I'm not sure the
> consequences of misinterpreting such sequences as single-pass are so
> terrible. Still, I would prefer if we were looking for "__next__" instead
> of next().

I'm not actually suggesting this as a reliable way to detect re-iterable 
objects, it's more of an observation.  If you want something that can be 
relied upon for optimizations that would probably require a new __magic__ 
attribute. Any suggestions?

> Isn't that passive/active distinction illusory though? What about
> __getattr__ methods?

I can't believe that any static or semi-static typing system will be able to 
handle __getattr__ virtual attributes.  An object simply won't match a type 
predicate if any of the attributes checked by the predicate are virtual.

> > I don't see how a future typing system could be retrofitted
> > to Python otherwise (pssst, don't tell anyone, but I'm working on such a
> > system...)
> 
> Nifty! I'd love to get a preview, if possible. Types come into play at the
> Python/C++ boundary, and I'm interested in how our systems will interact
> (c.f. http://aspn.activestate.com/ASPN/Mail/Message/types-sig/1222793)

I don't know what you're talking about.  :-)

	Oren