def _invalidFileDescriptor(self):
        """
        Get an invalid file descriptor.

        @return: An integer which is not a valid file descriptor at the time of
            this call.  After any future system call which allocates a new file
            descriptor, there is no guarantee the returned file descriptor will
            still be invalid.
        """
        fd = self.system.open(b"/dev/net/tun", os.O_RDWR)
        self.system.close(fd)
        return fd

        Get an invalid file descriptor.

        @return: An integer which is not a valid file descriptor at the time of
            this call.  After any future system call which allocates a new file
            descriptor, there is no guarantee the returned file descriptor will
            still be invalid.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/pair/test/test_tuntap.py