code: def _call_zadd(self, command, *args, **kwargs):
          
          if self.indexable:
              keys = []
              if args:
                  if len(args) % 2 != 0:
                      raise RedisError("ZADD requires an equal number of "
                                       "values and scores")
                  keys.extend(args[1::2])
              keys.extend(kwargs)  # add kwargs keys (values to index)
              self.index(keys)
          return self._traverse_command(command, *args, **kwargs)
docs: We do the same computation of the zadd method of StrictRedis to keep keys
          to index them (instead of indexing the whole set)
          Members (value/score) can be passed:
              - in *args, with score followed by the value, 0+ times (to respect
                the redis order)
              - in **kwargs, with value as key and score as value
          Example: zadd(1.1, 'my-key', 2.2, 'name1', 'name2', name3=3.3, name4=4.4)