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

[Guido van Rossum]

> On Fri, 12 Jul 2002, Guido van Rossum wrote:
> > I don't see what's wrong with the file object.  Iterating over a file
> > changes the file's state, that's just a fact of life.

[Ping]
> That's exactly the point.  Iterators and containers are different.
> Walking over a container shouldn't mutate it, whereas an iterator
> has mutable state independent of the container.
> 
> The key problem is that the file's __iter__ method returns something
> whose state depends on the file, thus breaking this expectation.
> Either __iter__ should be implemented to fulfill its commitment, or
> there shouldn't be an __iter__ method on files at all.

What commitment?

Iterators don't have to have an undelying container!  (E.g. generators.)

> I'm not suggesting that the semantics of files themselves are "broken"
> or have a "wart" that needs to be fixed -- merely that we should decide
> on a place for files to live in our world of containers and iterators,
> so we can set and maintain consistent expectations.

What are your expectations?  I think that both file.__iter__()
returning file (as it does with Oren's patch) or file.__iter__()
returning an xreadlines object (as it still does in CVS) are fine as
far as reasonable expectations for iterators go.

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