'Returns the upload ID for the resumable upload, or None if the upload
has not yet started.'

def get_upload_id(self):

    delim = '?upload_id='
    if (self.tracker_uri and (delim in self.tracker_uri)):
        return self.tracker_uri[(self.tracker_uri.index(delim) + len(delim)):]
    else:
        return None
