code: def _writtable(self, watcher, events):
          
          try:
              sent = self.sock.send(bytes(self.write_buffer))
              self.write_buffer = self.write_buffer[sent:]
              if len(self.write_buffer) == 0:
                  self.write_watcher.stop()
                  self.write = self.unbuffered_write
          except EnvironmentError as e:
              self._close(e)
docs: Called by the pyev watcher (self.write_watcher) whenever the socket
          is writtable.
  
          Calls send using the userspace buffer (self.write_buffer) and checks
          for errors. If there are no errors then continue on as before.
          Otherwise closes the socket and calls close_cb with the error.