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

[Andrew Koenig]

David> I'm with you on the desire to have a way to get a
David> multipass-iterator-or-error in one swell foop. That says "I
David> want to iterate over this thing without changing it". I still
David> think that hasattr(iter(x), '__copy__') is a pretty clean way
David> to do that, despite the fact that it potentially creates an
David> iterator (which some people apparently view as too heavyweight
David> as an introspection step).

In particular, creating an iterator had better not be a destructive operation.

David> However, I don't see any point in trying to define a protocol
David> for every different possible iteration view of a thing. Dicts
David> have keys, values, and items. Trees have breadth-first,
David> depth-first, inorder, postorder, blah, blah, blah. There are
David> just too many of these, and they're all different.

I wasn't suggesting defining a protocol for every possible iteration
view.  I was raising the question of whether multi-pass iteration
was likely to be a common enough operation that it is worth defining
a protocol for it, while leaving the door open to defining protocols
for others should it turn out to be desirable to do so.