code: def _set_unknown_flag(self, name, value):
      
      setter = self.__dict__['__set_unknown']
      if setter:
        try:
          setter(name, value)
          return value
        except (TypeError, ValueError):  # Flag value is not valid.
          raise _exceptions.IllegalFlagValueError(
              '"{1}" is not valid for --{0}' .format(name, value))
        except NameError:  # Flag name is not valid.
          pass
      raise _exceptions.UnrecognizedFlagError(name, value)
docs: Returns value if setting flag |name| to |value| returned True.
  
      Args:
        name: str, name of the flag to set.
        value: Value to set.
  
      Returns:
        Flag value on successful call.
  
      Raises:
        UnrecognizedFlagError
        IllegalFlagValueError