如何使用TextIO读取文本文件并返回额外的输入字段？

import apache_beam as beam
from apache_beam.io import ReadFromText

with beam.Pipeline() as pipeline:

    text_data = (pipeline
                 | 'Read from text file' >> ReadFromText('file_path.txt'))

    additional_fields = (text_data
                        | 'Add additional fields' >> beam.ParDo(AddFieldsFn()))

class AddFieldsFn(beam.DoFn):
    def process(self, element):
        # 处理返回的数据并添加额外的字段
        # ...
        # 返回处理后的数据
        return processed_element

    additional_fields | 'Write output' >> WriteToText('output.txt')