[Python-ideas] Introduce collections.Reiterable

[Neil Girdhar]

I appreciate the discussion illuminating various aspects of this I hadn't
considered. Finally, what I think I want is for
* all sequences
* all views
* numpy arrays
to answer yes to reiterable, and
* all generators
to answer no to reiterable.

Best, Neil

On Fri, Sep 20, 2013 at 10:12 PM, Stephen J. Turnbull <stephen at xemacs.org>wrote:

> Terry Reedy writes:
>
>  > Dismissing legal code as 'pathological', as more than one person has,
>  > does not cut it as a design principle.
>
> But you don't even need to write a class with __getitem__() to get
> that behavior.
>
> >>> l = [11, 12, 13]
> >>> for i in l:
> ...  print(i)
> ...  if i%2 == 0:
> ...   l.remove(i)
> ...
> 11
> 12
> >>> l
> [11, 13]
> >>>
>
> Of course the iteration itself is probably buggy (ie, the writer
> didn't mean to skip printing '13'), but in general iterables can
> change themselves.
>
> Neil himself seems to be of two minds about such cases.  On the one
> hand, he said the above behavior is built in to list, so it's
> acceptable to him.  (I think that's inconsistent: I would say the
> property of being completely consumed is built in to iterator, so it
> should be acceptable, too.)  On the other hand, he's defined a
> reiterable as a collection that when iterated produces the same
> objects in the same order.
>
> Maybe what we really want is for copy.deepcopy to do the right thing
> with iterables.  Then code that doesn't want to consume consumable
> iterables can do a deepcopy (including replication of the closed-over
> state of __next__() for iterators) before iterating.
>
> Or perhaps the right thing is a copy.itercopy that creates a new
> composite object as a shallow copy of everything except that it clones
> the state of __next__() in case the object was an iterator to start
> with.
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