code: def verify(self, flag_values):
      
      param = self._get_input_to_checker_function(flag_values)
      if not self.checker(param):
        raise _exceptions.ValidationError(self.message)
docs: Verifies that constraint is satisfied.
  
      flags library calls this method to verify Validator's constraint.
  
      Args:
        flag_values: flags.FlagValues, the FlagValues instance to get flags from.
      Raises:
        Error: Raised if constraint is not satisfied.