def __init__(self, callback, keys):
        self._max = len(keys)
        assert self._max > 1, 'need at least two sources'
        self._queues = dict((k, [None, deque(), k]) for k in keys)
        self._active = 0

        self._callback = callback

        self._key_getter = itemgetter(0)
        self._grpos_getter = itemgetter(0)
dataset/ETHPy150Open arkadini/twimp/twimp/utils.py/FrameSorter.__init__/original