<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 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_))