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

[Guido van Rossum]

> I'm thinking, in part, about how one might translate some of the C++
> standard-library algorithms into Python.  If that translation requires
> that the user always supply the original container, rather than using
> iterators only, then some algorithms become harder to express or less
> ueful.

Indeed.  There's a whole slew of interesting things you can do with
iterators that means you won't have a container, only an iterator.

For example, you can define "iterator algebra" functions that take
iterators and return iterators.  A simple example is this generator,
which yields alternating elements of a given iterator.

def alternating(it):
    while 1:
        yield it.next()
        it.next()

The nice thing is that you can combine these easily.  For example
alternating(alternating(it)) would yield every 4th element.

It would be a pity if the results of iterator algebra operations would
not be acceptable to Andrew's proposed algorithm library.

--Guido van Rossum (home page: http://www.python.org/~guido/)