code: def set_exit_callback(self, callback: Callable[[int], None]) -> None:
          
          self._exit_callback = callback
          Subprocess.initialize()
          Subprocess._waiting[self.pid] = self
          Subprocess._try_cleanup_process(self.pid)
docs: Runs ``callback`` when this process exits.
  
          The callback takes one argument, the return code of the process.
  
          This method uses a ``SIGCHLD`` handler, which is a global setting
          and may conflict if you have other libraries trying to handle the
          same signal.  If you are using more than one ``IOLoop`` it may
          be necessary to call `Subprocess.initialize` first to designate
          one ``IOLoop`` to run the signal handlers.
  
          In many cases a close callback on the stdout or stderr streams
          can be used as an alternative to an exit callback if the
          signal handler is causing a problem.
  
          Availability: Unix