def base64_user_pass(self):
        """
        Composes a basic http auth string, suitable for use with the
        _replicator database, and other places that need it.

        :returns: Basic http authentication string
        """
        if self._username is None or self._password is None:
            return None

        hash_ = base64.urlsafe_b64encode(bytes_("{username}:{password}".format(
            username=self._username,
            password=self._password
        )))
        return "Basic {0}".format(unicode_(hash_))
Composes a basic http auth string, suitable for use with the
        _replicator database, and other places that need it.

        :returns: Basic http authentication string