def __init__(self, callbacks, threshold=<NUM_LIT> * <NUM_LIT>):
self._callbacks = callbacks<EOL>self._threshold = threshold<EOL>self._bytes_seen = <NUM_LIT:0><EOL>
Aggregates progress updates for every provided progress callback

        :type callbacks: A list of functions that accepts bytes_transferred
            as a single argument
        :param callbacks: The callbacks to invoke when threshold is reached

        :type threshold: int
        :param threshold: The progress threshold in which to take the
            aggregated progress and invoke the progress callback with that
            aggregated progress total