python - CSV 行附加 : DictWriter always writes from first column

Question

输入文件:

$ cat test.csv 
company,spread,cat1,cat2,cat3
A,XYZ,32,67,0
B,XYZ,43,0,432
C,XYZ,32,76,32
D,XYZ,454,87,43
E,XYZ,0,0,65
F,XYZ,0,0,7

预期的 CSV 输出(对 cat1、cat2 和 cat3 列求和并附加总和。):

$ cat test.csv 
company,spread,cat1,cat2,cat3
A,XYZ,32,67,0
B,XYZ,43,0,432
C,XYZ,32,76,32
D,XYZ,454,87,43
E,XYZ,0,0,65
F,XYZ,0,0,7
,,561,230,579

代码:

import csv

all_keys = ['cat1', 'cat2', 'cat3']
default_values = {i: 0 for i in all_keys}

def read_csv():
    with open('test.csv', 'r') as f:
        reader = csv.DictReader(f)
        yield from reader

for row in read_csv():
    for i in all_keys:
        default_values[i] += int(row[i])

with open('test.csv', 'a') as w:
    writer = csv.DictWriter(w, fieldnames=all_keys)
    writer.writerow(default_values)

实际输出:

$ cat test.csv 
company,spread,cat1,cat2,cat3
A,XYZ,32,67,0
B,XYZ,43,0,432
C,XYZ,32,76,32
D,XYZ,454,87,43
E,XYZ,0,0,65
F,XYZ,0,0,7
561,230,579

问题:

csv.DictWriter 未附加具有正确列对齐的行。我知道我有 5 列，但我只提供 3 列的值。但我认为这是 DictWriter，它只会将值附加到匹配的列标题。如果我打开我的 Actual Output CSV，很明显列没有对齐:

Answer 1

您应该在 fieldnames 中包含前两个列的名称:

with open('test.csv', 'a') as w:
    writer = csv.DictWriter(w, fieldnames=['company', 'spread']+all_keys)
    writer.writerow(default_values)

如果键在字典中不可用，空值将写入前两列。