def test_connectionSerial(self):
        """
        Each L{FakeTransport} receives a serial number that uniquely identifies
        it.
        """
        a = FakeTransport(object(), True)
        b = FakeTransport(object(), False)
        self.assertIsInstance(a.serial, int)
        self.assertIsInstance(b.serial, int)
        self.assertNotEqual(a.serial, b.serial)

        Each L{FakeTransport} receives a serial number that uniquely identifies
        it.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/test/test_iosim.py