diff_iterators.py

def diff_iterators(iter1, iter2):
    """
    Yields every value that differs between two iterators, assuming they're ordered the same.

    ie, given
        iter1 = iter([1, 2, 5, 7, 9, 10, 12])
        iter2 = iter([1, 3, 4, 5, 6, 9, 11])
    yields:
        2, 3, 4, 6, 7, 10, 11, 12
    """
    try:
        blob1 = next(iter1)
    except StopIteration:
        # finished iter1? the rest of iter2 is a diff
        yield from iter2
        return
    try:
        blob2 = next(iter2)
    except StopIteration:
        # finished iter2? the rest of iter1 is a diff, including blob1 we just read
        yield blob1
        yield from iter1
        return
    while True:
        try:
            # yield from iter1 until we get an item that's >= blob2
            while blob1 < blob2:
                yield blob1
                blob1 = next(iter1)
        except StopIteration:
            # finished iter1? the rest of iter2 is a diff, including blob2 we just read
            yield blob2
            yield from iter2
            return
        try:
            while blob2 < blob1:
                yield blob2
                blob2 = next(iter2)
        except StopIteration:
            # finished iter2? the rest of iter1 is a diff, including blob1 we just read
            yield blob1
            yield from iter1
            return
        if blob1 == blob2:
            # since these blobs are the same, advance both iterators to the next item.
            try:
                blob1 = next(iter1)
            except StopIteration:
                # finished iter1? the rest of iter2 is a diff
                yield from iter2
                return
            try:
                blob2 = next(iter2)
            except StopIteration:
                # finished iter2? the rest of iter1 is a diff, including blob1 we just read
                yield blob1
                yield from iter1
                return