def _settings_checker(self, required_settings=None, accept_none=True):
        """
        Take a list of required _settings dictionary keys
        and make sure they are set. This can be added to a custom
        constructor in a subclass and tested to see if it returns ``True``.

        :arg list required_settings: A list of required keys to look for.
        :arg bool accept_none: Boolean set to True if None is an acceptable
                               setting. Set to False if None is not an
                               acceptable setting.

        :returns: * bool ``True`` if all required settings exist, OR
                  * str <key name> for the first key missing from _settings.
        """
        if required_settings is not None:
            for keyname in required_settings:
                if keyname not in self._settings:
                    return keyname
                if accept_none is False and self._settings[keyname] is None:
                    return keyname
        return True