def get_filenames(cls, mapreduce_state):
    """See parent class."""
    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