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

[Ka-Ping Yee]

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.

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.

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.


-- ?!ng