Saves mapping from submission IDs to original filenames.

    This mapping is saved as CSV file into target directory.

def save_id_to_path_mapping(self):
    """Saves mapping from submission IDs to original filenames.

    This mapping is saved as CSV file into target directory.
    """
    if not self.id_to_path_mapping:
      return
    with open(self.local_id_to_path_mapping_file, 'w') as f:
      writer = csv.writer(f)
      writer.writerow(['id', 'path'])
      for k, v in sorted(iteritems(self.id_to_path_mapping)):
        writer.writerow([k, v])
    cmd = ['gsutil', 'cp', self.local_id_to_path_mapping_file,
           os.path.join(self.target_dir, 'id_to_path_mapping.csv')]
    if subprocess.call(cmd) != 0:
      logging.error('Can\'t copy id_to_path_mapping.csv to target directory')