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

[Guido van Rossum]

> I do think there is some potential for errors caused by
> misunderstandings about whether or not "for x in y" is destructive.
> That's the thing that worries me the most.  I think this is the main
> reason why the old practice of abusing __getitem__ was bad, and thus
> helped to motivate iterators in the first place.  It seems serious
> enough that migrating to something that distinguishes
> destructive-for from non-destructive-for could indeed be worth the
> cost.

I'm not sure I understand this (this seems to be my week for not
understanding what people write :-( ).

First of all, I'm not sure what exactly the issue is with destructive
for-loops.  If I have a function that contains a for-loop over its
argument, and I pass iter(x) as the argument, then the iterator is
destroyed in the process, but x may or may not be, depending on what
it is.  Maybe the for-loop is a red herring?  Calling next() on an
iterator may or may not be destructive on the underlying "sequence" --
if it is a generator, for example, I would call it destructive.

Perhaps you're trying to assign properties to the iterator abstraction
that aren't really there?

Next, I'm not sure how renaming next() to __next__() would affect the
situation w.r.t. the destructivity of for-loops.  Or were you talking
about some other migration?

--Guido van Rossum (home page: http://www.python.org/~guido/)