def test_lookupSubsystem(self):
        """
        When a client requests a subsystem, the SSHSession object should get
        the subsystem by calling avatar.lookupSubsystem, and attach it as
        the client.
        """
        ret = self.session.requestReceived(
            b'subsystem', common.NS(b'TestSubsystem') + b'data')
        self.assertTrue(ret)
        self.assertIsInstance(self.session.client, protocol.ProcessProtocol)
        self.assertIs(self.session.client.transport.proto,
                      self.session.avatar.subsystem)

        When a client requests a subsystem, the SSHSession object should get
        the subsystem by calling avatar.lookupSubsystem, and attach it as
        the client.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/conch/test/test_session.py