def addWriter(self, writer):
        """Add a FileDescriptor for notification of data available to write.
        """
        fd = writer.fileno()
        if fd not in self._writes:
            self._selectables[fd] = writer
            self._writes[fd] =  1
            self._updateRegistration(fd)
Add a FileDescriptor for notification of data available to write.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/internet/pollreactor.py