def zipper(list1, list2):
    """
    Zip two lists, alternating values. I.e.: zipper([1,2,3], [4,5,6]) == [1,4,2,5,3,6]
    :return:
    """
    result = [None] * (len(list1) + len(list2))
    result[::2] = list1
    result[1::2] = list2
    return result