A function decorator for defining a flag validator.

    flag_name, message='Flag validation failed', flag_values=_flagvalues.FLAGS

Registers the decorated function as a validator for flag_name, e.g.

@flags.validator('foo') def _CheckFoo(foo): ...

See register_validator() for the specification of checker function.


  • flag_name: str, name of the flag to be checked.
  • message: str, error text to be shown to the user if checker returns False. If checker raises flags.ValidationError, message from the raised error will be shown.
  • flag_values: flags.FlagValues, optional FlagValues instance to validate against.


A function decorator that registers its function argument as a validator.


  • AttributeError: Raised when flag_name is not registered as a valid flag name.