A function decorator for defining a flag validator.

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.