像这样编写CSV有点复杂，字节被写入文件，添加另一列需要 * 插入 * 字节，这通常需要保存整个新文件。
你应该做的是创建元组或列表来表示结果行，然后一个一个地写，因为你的代码片段不能执行，所以让我来演示一下：

import csv

data = [(1, 2), (3, 4), (5, 6)]
data_sum = [sum(i) for i in data]  # [3, 7, 11]

with open('/tmp/sum.csv', 'w') as csv_file:
    csv_writer = csv.writer(csv_file)
    csv_writer.writerow(['a', 'b', 'sum'])
    for ((a, b), s) in zip(data, data_sum):
        csv_writer.writerow([a, b, s])

或者将for循环替换为cvs.Writer.writerows()调用：

csv_writer.writerows((a, b, s) for ((a, b), s) in zip(data, data_sum))

import csv

with open('file.csv', 'w', newline='') as csvfile:    
    writer = csv.writer(csvfile)
 
    for row in rows:
        writer.writerow(row)

    # Start a new output to a different column of the same file
    # by writing an empty row to the CSV file
    writer.writerow([])