def auth_property(name, doc=None):  # noqa: B902
        """A static helper function for subclasses to add extra authentication
        system properties onto a class::

            class FooAuthenticate(WWWAuthenticate):
                special_realm = auth_property('special_realm')

        For more information have a look at the sourcecode to see how the
        regular properties (:attr:`realm` etc.) are implemented.
        """

        def _set_value(self, value):
            if value is None:
                self.pop(name, None)
            else:
                self[name] = str(value)

        return property(lambda x: x.get(name), _set_value, doc=doc)