gistfile1.pyw

from itertools import chain, groupby

def as_pairs (xs, ys, grouper=lambda _: _):
    """
    >>> aordered = iter([1, 1, 2, 4, 6, 9])
    >>> bordered = iter([1, 3, 4, 5, 6, 7])
    >>> for a, b in as_pairs(aordered, bordered):
    ...     a, b
    ([1, 1], [1])
    ([2], None)
    (None, [3])
    ([4], [4])
    (None, [5])
    ([6], [6])
    (None, [7])
    ([9], None)
    """
    xs = chain(xs, [None])
    ys = chain(ys, [None])
    xnext = groupby(xs, grouper).next
    ynext = groupby(ys, grouper).next

    x, xgroup = xnext()
    y, ygroup = ynext()
    while (x,y) != (None, None):
        if x == y:
            yield list(xgroup), list(ygroup)
            x, xgroup = xnext()
            y, ygroup = ynext()
        elif y is None or (x is not None and x < y):
            yield list(xgroup), None
            x, xgroup = xnext()
        else:
            yield (None, list(ygroup))
            y, ygroup = ynext()

if __name__ == "__main__":
    import doctest
    doctest.testmod()