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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def _empty_notification(self):
 """ empty and return list of message notification """

        sess = cherrypy.session
        username = sess.get(SESSION_KEY, None)
        if username in self.notifications:
            ret = self.notifications[username]
        else:
            ret = []
        self.notifications[username] = []
        return ret