def setUp(self):
        """
        Save the current SIGCHLD handler as reported by L{signal.signal} and
        the current file descriptor registered with L{installHandler}.
        """
        handler = signal.getsignal(signal.SIGCHLD)
        if handler != signal.SIG_DFL:
            self.signalModuleHandler = handler
            signal.signal(signal.SIGCHLD, signal.SIG_DFL)
        else:
            self.signalModuleHandler = None

        self.oldFD = installHandler(-1)

        if self.signalModuleHandler is not None and self.oldFD != -1:
            msg("Previous test didn't clean up after its SIGCHLD setup: %r %r"
                % (self.signalModuleHandler, self.oldFD))

        Save the current SIGCHLD handler as reported by L{signal.signal} and
        the current file descriptor registered with L{installHandler}.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/internet/test/test_sigchld.py