Return a representation of the id map as a string. This string is
        properly formatted to be written in '/etc/subuid' or '/etc/subgid'.

def write_string(self):
        """
        Return a representation of the id map as a string. This string is
        properly formatted to be written in '/etc/subuid' or '/etc/subgid'.
        """

        map_as_str = []
        for name, id_range_set in self.__map.items():
            for id_range in id_range_set:
                map_as_str.append(
                        name + ':' +
                        str(id_range.first) + ':' +
                        str(id_range.count) + '\n'
                    )

        # Remove trailing newline
        if len(map_as_str) > 0:
            map_as_str[-1] = map_as_str[-1][:-1]

        return ''.join(map_as_str)