code: def get_filenames(cls, mapreduce_state):
      
      shards = mapreduce_state.mapreduce_spec.mapper.shard_count
      filenames = []
      for _ in range(shards):
        filenames.append([None] * shards)
      shard_states = model.ShardState.find_all_by_mapreduce_state(mapreduce_state)
      for x, shard_state in enumerate(shard_states):
        shard_filenames = shard_state.writer_state["shard_filenames"]
        for y in range(shards):
          filenames[y][x] = shard_filenames[y]
      return filenames
docs: See parent class.