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

[Andrew Koenig]

>> However the purpose my suggestion of __multiter__ was not to use it to
>> test for multiple iteration, but to enable a container to be able to
>> yield either a single or a multiple iterator on request.

David> Why would you want that? Seems like a corner case at best.

You're right -- I wasn't thinking clearly.

What I meant to say was that I would like a program that expects
to be able to use a multiple iterator to be able to say so simply
and efficiently in code.  For example:

    for i in multiter(x):
       // whatever

I would like this to fail cleanly if x does not support multiple
iterators.

>> A data structure that supports several different kinds of iteration
>> has to provide that support somehow.  What's your suggestion?

David> class DataStructure(object):
David>     def __init__(self):
David>         self._numbers = range(10);
David>         self._names = [ str(x) for x in range(10) ];

David>     names = property(lambda self: iter(self._names))
David>     numbers = property(lambda self: iter(self._numbers))

David> x = DataStructure();
David> for y in x.names:
David>     print repr(y),

David> print

David> for y in x.numbers:
David>     print repr(y),

David> [Y'know, Python is great. That worked the first time I ran it.]

I don't understand how this code answers my question.
You've asked for iterators over two different data structures.
What I was asking was, for example, how one might arrange for a single
tree to yield either a depth-first or breadth-first iterator.