def connectionLost(self, reason):
                """
                Notify observers that the server side of the connection has
                ended.
                """
                msg('Disconnecter.connectionLost')
                serverLost.callback(None)
                msg('serverLost called back')

                Notify observers that the server side of the connection has
                ended.
                
stackoverflow/venv/lib/python3.6/site-packages/twisted/test/test_tcp.py