Ensures that flags are not None during program execution.

  Recommended usage:

    if __name__ == '__main__':
      flags.mark_flags_as_required(['flag1', 'flag2', 'flag3'])
      app.run()

  Args:
    flag_names: Sequence[str], names of the flags.
    flag_values: flags.FlagValues, optional FlagValues instance where the flags
        are defined.
  Raises:
    AttributeError: If any of flag name has not already been defined as a flag.

def mark_flags_as_required(flag_names, flag_values=_flagvalues.FLAGS):
  """Ensures that flags are not None during program execution.

  Recommended usage:

    if __name__ == '__main__':
      flags.mark_flags_as_required(['flag1', 'flag2', 'flag3'])
      app.run()

  Args:
    flag_names: Sequence[str], names of the flags.
    flag_values: flags.FlagValues, optional FlagValues instance where the flags
        are defined.
  Raises:
    AttributeError: If any of flag name has not already been defined as a flag.
  """
  for flag_name in flag_names:
    mark_flag_as_required(flag_name, flag_values)