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

[David Abrahams]

Oren,

I like the direction this is going in, but I have some reservations about
any protocol which requires users to avoid using a simple method name like
next() on their own multi-pass sequence types unless they intend their
sequence to be treated as single-pass.

One other possibility: if x.__iter__() is x, it's a single-pass sequence. I
realize this involves actually invoking the __iter__ method and conjuring
up a new iterator, but that's generally a lightweight operation...

-Dave

From: "Oren Tirosh" <oren-py-d@hishome.net>

> There is no need for a new type of iterator. It's ok that iterators are
> disposable.  If I need multiple iterations I don't want to copy the
> iterator - I prefer to ask the original iterable object for a new
iterator.
> All I need is some way to know whether the iterable object (container)
can
> produce multiple iterators that generate the same sequence.
>
>   An object is re-iterable if its iterators do not modify its state.
>
> The iterator of an iterator is itself.  Calling the next method, by
> definition, modifies the internal state of an object. Therefore anything
> that has a next method is not re-iterable.
>
> "hasattr(obj,'__iter__') and hasattr(obj, 'next')" is a good signature of
> a non re-iterable object.  Unfortunately, the opposite is not true.  One
> iterable object in Python produces iterators that modify its state when
> their .next() method is called - the file object.
>
> I have just submitted a patch that makes a file into an iterator (i.e.
adds
> a .next method to files).  With this change all Python objects that have
> an __iter__ method and no next method produce iterators that do not
modify
> the container.  Another possibility would be to make file iterators that
> use seek or re-open the file to avoid modifying the file position of the
> parent file object.  I don't think that would be a good idea because
files
> can be devices, pipes or sockets which are not seekable.
>
> I think it may be a good idea to add a note to the documentation pages
> about the iterator protocol that the iterators of a container should not
> modify the state of the container. If you think they must it's probably
> a good sign that your 'container' is not really a container and maybe it
> should be an iterator rather than produce iterators of itself.
>
> Oren
>
>
>
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