def test_pidFile(self):
        """
        A lockfile is created and locked when L{IReactorUNIX.listenUNIX} is
        called and released when the Deferred returned by the L{IListeningPort}
        provider's C{stopListening} method is called back.
        """
        filename = self.mktemp()
        serverFactory = MyServerFactory()
        serverConnMade = defer.Deferred()
        serverFactory.protocolConnectionMade = serverConnMade
        unixPort = reactor.listenUNIX(filename, serverFactory, wantPID=True)
        self.assertTrue(lockfile.isLocked(filename + ".lock"))

        # XXX This part would test something about the checkPID parameter, but
        # it doesn't actually.  It should be rewritten to test the several
        # different possible behaviors.  -exarkun
        clientFactory = MyClientFactory()
        clientConnMade = defer.Deferred()
        clientFactory.protocolConnectionMade = clientConnMade
        reactor.connectUNIX(filename, clientFactory, checkPID=1)

        d = defer.gatherResults([serverConnMade, clientConnMade])
        def _portStuff(args):
            serverProtocol, clientProto = args

            # Incidental assertion which may or may not be redundant with some
            # other test.  This probably deserves its own test method.
            self.assertEqual(clientFactory.peerAddresses,
                             [address.UNIXAddress(filename)])

            clientProto.transport.loseConnection()
            serverProtocol.transport.loseConnection()
            return unixPort.stopListening()
        d.addCallback(_portStuff)

        def _check(ignored):
            self.assertFalse(lockfile.isLocked(filename + ".lock"), 'locked')
        d.addCallback(_check)
        return d

        A lockfile is created and locked when L{IReactorUNIX.listenUNIX} is
        called and released when the Deferred returned by the L{IListeningPort}
        provider's C{stopListening} method is called back.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/test/test_unix.py