def diff_lists(list1,list2, option=None):
    """
    if option equal 'and', return a list of items which are in both list1
    and list2. Otherwise, return a list of items in list1 but not in list2.
    """

    if option and option == 'and':
        return [x for x in list1 if x in list2] 
    else:
        return [x for x in list1 if x not in list2]