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.