code: def Instance(expected, message="Not an instance of {}"):
      
      @wraps(Instance)
      def built(value):
          if not isinstance(value, expected):
              raise Error(message.format(expected.__name__))
          return value
      return built
docs: Creates a validator that checks if the given value is an instance of
      ``expected``.
  
      A custom message can be specified with ``message``.