'Initialize the ExportProgressThread instance.
Args:
exporter: An Exporter instance for the download.
progress_queue: A Queue used for tracking progress information.
progress_db: The database for tracking progress information; should
be an instance of ProgressDatabase.
result_db: The database for holding exported entities; should be an
instance of ResultDatabase.'

def __init__(self, exporter, progress_queue, progress_db, result_db):

    _ProgressThreadBase.__init__(self, progress_queue, progress_db)
    self.exporter = exporter
    self.existing_count = result_db.existing_count
    self.result_db = result_db
