def beam_energy(event):
    """
    Gets the beam energy for an event retrieved from Karabo at XFEL.

    Karabo events do not currently contain beam energy information. This function takes
    it from the configuration file, specifically from the
    'fallback_beam_energy_in_eV' entry in the 'DataRetrievalLayer' parameter
    group.

    Arguments:

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

    Returns:

        float: the energy of the beam in eV.
    """
    return event.framework_info['beam_energy']
