def invoke_progress_callbacks(callbacks, bytes_transferred):
<EOL>if bytes_transferred:<EOL><INDENT>for callback in callbacks:<EOL><INDENT>callback(bytes_transferred=bytes_transferred)<EOL><DEDENT><DEDENT>
Calls all progress callbacks

    :param callbacks: A list of progress callbacks to invoke
    :param bytes_transferred: The number of bytes transferred. This is passed
        to the callbacks. If no bytes were transferred the callbacks will not
        be invoked because no progress was achieved. It is also possible
        to receive a negative amount which comes from retrying a transfer
        request.