def test_killExited(self):
        """
        L{process.Process.signalProcess} raises L{error.ProcessExitedAlready}
        if the process has exited.
        """
        self.mockos.child = False
        cmd = b'/mock/ouch'
        p = TrivialProcessProtocol(None)
        proc = reactor.spawnProcess(p, cmd, [b'ouch'], env=None, usePTY=False)
        # We didn't specify a waitpid value, so the waitpid call in
        # registerReapProcessHandler has already reaped the process
        self.assertRaises(error.ProcessExitedAlready,
                          proc.signalProcess, "KILL")

        L{process.Process.signalProcess} raises L{error.ProcessExitedAlready}
        if the process has exited.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/test/test_process.py