def beam_energy(event):
    """
    Gets the beam energy for a data event retrieved from the filesystem.

    Files written by detectors don't 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']
