@staticmethod
def _diff_lists(list_1, list_2):
    """
        Calculate the set difference between two lists.

        :param list_1: (list) a list of asset indices
        :param list_2: (list) another list of asset indices
        :return: (list) set difference between the two input lists
        """
    return list(set(list_1) - set(list_2))
