def frame_id(event):
    """
    Gets a unique identifier for a data frame retrieved from Karabo at XFEL.

    Returns a label that unambiguosly identifies, within an event, the frame currently
    being processed. At the European XFEL facility, the index of the frame within its
    train is used as identifier.

    Arguments:

         event (:class:`~onda.utils.data_event.DataEvent`): an object storing the event
            data.

    Returns:

        str: a unique frame identifier (within an event).
    """
    return str(event.current_frame)
