def to_json(self):
    """Serializes this MapperSpec into a json-izable object."""
    result = {
        "mapper_handler_spec": self.handler_spec,
        "mapper_input_reader": self.input_reader_spec,
        "mapper_params": self.params,
        "mapper_shard_count": self.shard_count
    }
    if self.output_writer_spec:
      result["mapper_output_writer"] = self.output_writer_spec
    return result
Serializes this MapperSpec into a json-izable object.