code: def add_reader(self, fd, callback):
          " Add read file descriptor to the event loop. "
          fd = fd_to_int(fd)
          self._read_fds[fd] = callback
          self.selector.register(fdf add_reader(self, fd, callback):
          " Add read file descriptor to the event loop. "
          fd = fd_to_int(fd)
          self._read_fds[fd] = callback
          self.selector.register(fd)
docs: Add read file descriptor to the event loop.