def get_fraction_dead(dfo,
                      dead_cell_columns=['corpse_count', 'cell_count__dead'], 
                      total_cell_columns=['cell_count', 'cell_count__dead',
                                          'corpse_count']):
    """Appends column for fraction of dead cells

    Parameters
    ----------
    dfo : pandas dataframe
    dead_cell_columns : list of str
        list of column names corresponding to dead cells
    total_cell_columns : list of str
        list of column names corresponding to total cell counts

    Returns
    -------
    dfo2 : pandas dataframe    
    """
    dfo2 = dfo.copy()
    dfo2['fraction_dead'] = dfo2[dead_cell_columns].sum(axis=1).div(
        dfo2[total_cell_columns].sum(axis=1))
    return dfo2