Efficient, lazy, iterator, python pairing

gistfile1.py

from itertools import chain, tee


def pairs(iterator):
    """
    Returns the items in the iterator pairwise, like so;

    >>> list(pairs([0, 1, 2]))
    [(0, 1), (1, 2)]
    """
    first, second = tee(iterator)
    ret = zip(chain([None], second), first)

    # this is a generator so that execution doesn't begin until the user
    # starts consuming us, and so that we then don't start consuming our
    # iterator until they ask us to

    next(ret)  # kill the first half-empty pair

    yield from ret