code: def start(check_time: int = 500) -> None:
      
      io_loop = ioloop.IOLoop.current()
      if io_loop in _io_loops:
          return
      _io_loops[io_loop] = True
      if len(_io_loops) > 1:
          gen_log.warning("tornado.autoreload started more than once in the same process")
      modify_times = {}  # type: Dict[str, float]
      callback = functools.partial(_reload_on_update, modify_times)
      scheduler = ioloop.PeriodicCallback(callback, check_time)
      scheduler.start()
docs: Begins watching source files for changes.
  
      .. versionchanged:: 5.0
         The ``io_loop`` argument (deprecated since version 4.1) has been removed.