Returns filename from a flagfile_str of form -[-]flagfile=filename.

    The cases of --flagfile foo and -flagfile foo shouldn't be hitting
    this function, as they are dealt with in the level above this
    function.

    Args:
      flagfile_str: str, the flagfile string.

    Returns:
      str, the filename from a flagfile_str of form -[-]flagfile=filename.

    Raises:
      Error: Raised when illegal --flagfile is provided.

def _extract_filename(self, flagfile_str):
    """Returns filename from a flagfile_str of form -[-]flagfile=filename.

    The cases of --flagfile foo and -flagfile foo shouldn't be hitting
    this function, as they are dealt with in the level above this
    function.

    Args:
      flagfile_str: str, the flagfile string.

    Returns:
      str, the filename from a flagfile_str of form -[-]flagfile=filename.

    Raises:
      Error: Raised when illegal --flagfile is provided.
    """
    if flagfile_str.startswith('--flagfile='):
      return os.path.expanduser((flagfile_str[(len('--flagfile=')):]).strip())
    elif flagfile_str.startswith('-flagfile='):
      return os.path.expanduser((flagfile_str[(len('-flagfile=')):]).strip())
    else:
      raise _exceptions.Error(
          'Hit illegal --flagfile type: %s' % flagfile_str)