diff_lists.py

def diff_lists(old_list, new_list):
    """
        >>> diff_lists([1,2,3], [2,3,4])  # removed 1, added 4
        [('-', 1), ('+', 4)]
        >>> diff_lists([1,2,3], [1,2,3])
        []
    """
    old = set(old_list)
    new = set(new_list)
    return [('-', e) for e in old-new] + [('+', e) for e in new-old]


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