Difference between the column value of 2 rows

row_diff.py

def row_diff(rows, column_position):
    """
    gets the difference between 2 row values
    """
    # the column position parameter starts to count from 1
    # so have to make it start counting from 0
    column_position = column_position - 1

    results = []
    prev_value = None
    for row in rows:
        new_row = list(row)

        if prev_value:
            diff = new_row[column_position] - prev_value
            new_row.append(diff)
        else:
            new_row.append(0)

        prev_value = new_row[column_position]

        results.append(new_row)

    return results


def main():
    # assume the first item in a row is an id or something
    rows = (
        (1, 20),
        (2, 45),
        (3, 78),
        (4, 104),
    )

    results = row_diff(rows, column_position=2)

    print results

if __name__ == "__main__":
    main()