def MarkFlagAsRequired(flag_name, flag_values=FLAGS):
  """Ensure that flag is not None during program execution.

  Registers a flag validator, which will follow usual validator
  rules.
  Args:
    flag_name: string, name of the flag
    flag_values: FlagValues
  Raises:
    AttributeError: if flag_name is not registered as a valid flag name.
  """
  RegisterValidator(flag_name,
                    lambda value: value is not None,
                    message=flag_name % 'Flag --%s must be specified.',
                    flag_values=flag_values)
dataset/ETHPy150Open AppScale/appscale/AppServer/lib/python-gflags/gflags.py/MarkFlagAsRequired/swapped operands of `%`