Begins watching source files for changes.

    .. versionchanged:: 5.0
       The ``io_loop`` argument (deprecated since version 4.1) has been removed.

def start(check_time: int = 500) -> None:
    """Begins watching source files for changes.

    .. versionchanged:: 5.0
       The ``io_loop`` argument (deprecated since version 4.1) has been removed.
    """
    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()