Return a list of the top ``limit`` keys between ``name_start`` and
        ``name_end``

        Similiar with **Redis.KEYS**        

        .. note:: The range is (``name_start``, ``name_end``]. ``name_start``
           isn't in the range, but ``name_end`` is.

        :param string name_start: The lower bound(not included) of keys to be
         returned, empty string ``''`` means -inf
        :param string name_end: The upper bound(included) of keys to be
         returned, empty string ``''`` means +inf
        :param int limit: number of elements will be returned.
        :return: a list of keys
        :rtype: list
        
        >>> ssdb.keys('set_x1', 'set_x3', 10)
        ['set_x2', 'set_x3']
        >>> ssdb.keys('set_x ', 'set_xx', 3)
        ['set_x1', 'set_x2', 'set_x3']
        >>> ssdb.keys('set_x ', '', 3)
        ['set_x1', 'set_x2', 'set_x3', 'set_x4']
        >>> ssdb.keys('set_zzzzz ', '', )
        []

def keys(self, name_start, name_end, limit=10):
        """
        Return a list of the top ``limit`` keys between ``name_start`` and
        ``name_end``

        Similiar with **Redis.KEYS**        

        .. note:: The range is (``name_start``, ``name_end``]. ``name_start``
           isn't in the range, but ``name_end`` is.

        :param string name_start: The lower bound(not included) of keys to be
         returned, empty string ``''`` means -inf
        :param string name_end: The upper bound(included) of keys to be
         returned, empty string ``''`` means +inf
        :param int limit: number of elements will be returned.
        :return: a list of keys
        :rtype: list
        
        >>> ssdb.keys('set_x1', 'set_x3', 10)
        ['set_x2', 'set_x3']
        >>> ssdb.keys('set_x ', 'set_xx', 3)
        ['set_x1', 'set_x2', 'set_x3']
        >>> ssdb.keys('set_x ', '', 3)
        ['set_x1', 'set_x2', 'set_x3', 'set_x4']
        >>> ssdb.keys('set_zzzzz ', '', )
        []
        """
        limit = get_positive_integer('limit', limit)
        return self.execute_command('keys', name_start, name_end, limit)