def test_returnOldFD(self):
        """
        L{installHandler} returns the previously registered file descriptor.
        """
        read, write = self.pipe()
        oldFD = installHandler(write)
        self.assertEqual(installHandler(oldFD), write)

        L{installHandler} returns the previously registered file descriptor.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/internet/test/test_sigchld.py