def __init__(self, evt=None):
        """
        Args:
            evt(threading.Event): if None, will create one.
        """
        super(StoppableThread, self).__init__()
        if evt is None:
            evt = threading.Event()
        self._stop_evt = evt

        Args:
            evt(threading.Event): if None, will create one.
        
tensorpack/utils/concurrency.py