def setUp(self):
        """
        Create an in-memory I/O system and set up a L{TuntapPort} against it.
        """
        self.name = b"tun0"
        self.system = MemoryIOSystem()
        self.system.registerSpecialDevice(Tunnel._DEVICE_NAME, Tunnel)
        self.protocol = self.factory.buildProtocol(
            TunnelAddress(self.helper.TUNNEL_TYPE, self.name))
        self.reactor = FSSetClock()
        self.port = TuntapPort(
            self.name, self.protocol, reactor=self.reactor, system=self.system)

        Create an in-memory I/O system and set up a L{TuntapPort} against it.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/pair/test/test_tuntap.py