def frame_id(event):
    """
    Gets a unique identifier for a Pilatus data frame retrieved from files (or HiDRA).

    Returns a label that unambiguosly identifies, within an event, the frame currently
    being processed. For the Pilatus detector, the index of the frame within the event
    is used as identifier. However, each Pilatus event only contains one frame, so this
    function always returns the string "0".

    Arguments:

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

    Returns:

        str: a unique frame identifier (within an event).
    """
    del event
    return str(0)
