code: def prepare_subprocess_cmd(subprocess_cmd):
      
      help_cmd = subprocess_cmd + ['--helpfull']
      help_output = subprocess.run(help_cmd, stdout=subprocess.PIPE).stdout
      help_output = help_output.decode('ascii')
      if 'python' in subprocess_cmd[0]:
          valid_flags = parse_helpfull_output(help_output)
      else:
          valid_flags = parse_helpfull_output(help_output, regex=FLAG_HELP_RE_CC)
      parsed_flags = flags.FlagValues().read_flags_from_files(subprocess_cmd[1:])
  
      filtered_flags = filter_flags(parsed_flags, valid_flags)
      return [subprocess_cmd[0]] + filtered_flags
docs: Prepares a subprocess command by running --helpfull and masking flags.
  
      Args:
          subprocess_cmd: List[str], what would be passed into subprocess.call()
              i.e. ['python', 'train.py', '--flagfile=flags']
  
      Returns:
          ['python', 'train.py', '--train_flag=blah', '--more_flags']