def METHOD_NAME(self):
    if self.interface.multiple_select:
        store, itrs = self.selection.get_selected_rows()
        return [self.interface.data.index(store[itr][0].value) for itr in itrs]
    else:
        store, iter = self.selection.get_selected()
        if iter is None:
            return None
        return self.interface.data.index(store[iter][0].value)