def tearDown(self):
        """
        Restore whatever signal handler was present when setUp ran.
        """
        # If tests set up any kind of handlers, clear them out.
        installHandler(-1)
        signal.signal(signal.SIGCHLD, signal.SIG_DFL)

        # Now restore whatever the setup was before the test ran.
        if self.signalModuleHandler is not None:
            signal.signal(signal.SIGCHLD, self.signalModuleHandler)
        elif self.oldFD != -1:
            installHandler(self.oldFD)

        Restore whatever signal handler was present when setUp ran.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/internet/test/test_sigchld.py