def beam_energy(event):
    """
    Gets the beam energy for an event retrieved from HiDRA at Petra III.

    HiDRA events do not usually contain beam energy information. This function takes
    the beam energy value 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']
