def _reallyStartRunning(self):
        """
        Extend the base implementation by also installing signal handlers, if
        C{self._installSignalHandlers} is true.
        """
        ReactorBase._reallyStartRunning(self)
        if self._installSignalHandlers:
            # Make sure this happens before after-startup events, since the
            # expectation of after-startup is that the reactor is fully
            # initialized.  Don't do it right away for historical reasons
            # (perhaps some before-startup triggers don't want there to be a
            # custom SIGCHLD handler so that they can run child processes with
            # some blocking api).
            self._handleSignals()

        Extend the base implementation by also installing signal handlers, if
        C{self._installSignalHandlers} is true.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/internet/base.py