def run(self, table):
        new_column = []

        for row in table.rows:
            before = row[self._before_column_name]
            after = row[self._after_column_name]

            if before and after:
                new_column.append(after - before)
            else:
                new_column.append(None)

        return new_column
dataset/ETHPy150Open wireservice/agate/agate/computations/change.py/Change.run/original