Adds a constraint, which will be enforced during program execution.

The constraint is validated when flags are initially parsed, and after each change of the corresponding flag's value. Args: flag_name: str, name of the flag to be checked. checker: callable, a function to validate the flag. input - A single positional argument: The value of the corresponding flag (string, boolean, etc. This value will be passed to checker by the library). output - bool, True if validator constraint is satisfied. If constraint is not satisfied, it should either return False or raise flags.ValidationError(desired_error_message). 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. Raises: AttributeError: Raised when flag_name is not registered as a valid flag name.