def wakeUp(self):
        """Write one byte to the pipe, and flush it.
        """
        # We don't use fdesc.writeToFD since we need to distinguish
        # between EINTR (try again) and EAGAIN (do nothing).
        if self.o is not None:
            try:
                util.untilConcludes(os.write, self.o, b'x')
            except OSError as e:
                # XXX There is no unit test for raising the exception
                # for other errnos. See #4285.
                if e.errno != errno.EAGAIN:
                    raise
Write one byte to the pipe, and flush it.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/internet/posixbase.py