Returns the sum of elements of the sorted set stored at the specified
        key which have scores in the range [score_start,score_end].
        
        .. note:: The range is [``score_start``, ``score_end``]

        :param string name: the zset name
        :param int score_start: The minimum score related to keys(included),
         empty string ``''`` means -inf
        :param int score_end: The maximum score(included) related to keys,
         empty string ``''`` means +inf
        :return: the sum of keys in specified range
        :rtype: int
        
        >>> ssdb.zsum('zset_1', 20, 70)
        114
        >>> ssdb.zsum('zset_1', 0, 100)
        215
        >>> ssdb.zsum('zset_1', 2, 3)
        0

def zsum(self, name, score_start, score_end):
        """
        Returns the sum of elements of the sorted set stored at the specified
        key which have scores in the range [score_start,score_end].
        
        .. note:: The range is [``score_start``, ``score_end``]

        :param string name: the zset name
        :param int score_start: The minimum score related to keys(included),
         empty string ``''`` means -inf
        :param int score_end: The maximum score(included) related to keys,
         empty string ``''`` means +inf
        :return: the sum of keys in specified range
        :rtype: int
        
        >>> ssdb.zsum('zset_1', 20, 70)
        114
        >>> ssdb.zsum('zset_1', 0, 100)
        215
        >>> ssdb.zsum('zset_1', 2, 3)
        0
        """        
        score_start = get_integer_or_emptystring('score_start', score_start)
        score_end = get_integer_or_emptystring('score_end', score_end)
        return self.execute_command('zsum', name, score_start, score_end)