docs: 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)