csv DictReader 上的“工作”失败

Question

我正在编写一个脚本，我需要将 CSV 读入 DictReader，在字段上做一些工作（数据处理），然后通过 DictWriter 将 DictReader 输出到 csv。

如果我读取 CSV 然后写入 Dict，则该过程有效。

#Create the sample file
headers = ['Symbol', 'Price', 'Date', 'Time', 'Change', 'Volume']
rows = [{'Symbol':'AA', 'Price':39.48, 'Date':'6/11/2007',
          'Time':'9:36am', 'Change':-0.18, 'Volume':181800},
        {'Symbol':'AIG', 'Price': 71.38, 'Date':'6/11/2007',
          'Time':'9:36am', 'Change':-0.15, 'Volume': 195500},
        {'Symbol':'AXP', 'Price': 62.58, 'Date':'6/11/2007',
          'Time':'9:36am', 'Change':-0.46, 'Volume': 935000},
        ]

#Open sample file
with open('stocks.csv','w') as f:
    f_csv = csv.DictWriter(f, headers)
    f_csv.writeheader()
    f_csv.writerows(rows)

#Output the dict    
with open('stocks.csv', 'r') as file:
    csvread = csv.DictReader(file, delimiter=',')

    with open('out.csv', 'w') as out:
        headertowrite = ['Time', 'Symbol', 'NewColumn']
        writer = csv.DictWriter(out, headertowrite, extrasaction='ignore')
        writer.writeheader()
        writer.writerows(csvread)
#Works!

但是 - 如果我添加一个新列，我似乎会丢失 DictReader 中的所有数据：

headers = ['Symbol', 'Price', 'Date', 'Time', 'Change', 'Volume']
rows = [{'Symbol':'AA', 'Price':39.48, 'Date':'6/11/2007',
          'Time':'9:36am', 'Change':-0.18, 'Volume':181800},
        {'Symbol':'AIG', 'Price': 71.38, 'Date':'6/11/2007',
          'Time':'9:36am', 'Change':-0.15, 'Volume': 195500},
        {'Symbol':'AXP', 'Price': 62.58, 'Date':'6/11/2007',
          'Time':'9:36am', 'Change':-0.46, 'Volume': 935000},
        ]

with open('stocks.csv','w') as f:
    f_csv = csv.DictWriter(f, headers)
    f_csv.writeheader()
    f_csv.writerows(rows)

with open('stocks.csv', 'r') as file:
    csvread = csv.DictReader(file, delimiter=',')


    for row in csvread:
        row['NewColumn'] = '1'

    with open('out.csv', 'w') as out:
        headertowrite = ['Time', 'Symbol', 'NewColumn']
        writer = csv.DictWriter(out, headertowrite, extrasaction='ignore')
        writer.writeheader()
        writer.writerows(csvread)
#Out.csv is blank!

有没有一种方法可以在编写之前对 DictReader 执行工作？

Answer 1

由于 csvread 已被您的第一个 for 循环完全消耗，我们的 with 语句将输出一个空白文件，您可能会注意到。

您应该：

--> 将行保存到变量中

import csv
headers = ['Symbol', 'Price', 'Date', 'Time', 'Change', 'Volume']
rows = [{'Symbol':'AA', 'Price':39.48, 'Date':'6/11/2007',
         'Time':'9:36am', 'Change':-0.18, 'Volume':181800},
        {'Symbol':'AIG', 'Price': 71.38, 'Date':'6/11/2007',
         'Time':'9:36am', 'Change':-0.15, 'Volume': 195500},
        {'Symbol':'AXP', 'Price': 62.58, 'Date':'6/11/2007',
         'Time':'9:36am', 'Change':-0.46, 'Volume': 935000}]

with open('stocks.csv','w') as f:
    f_csv = csv.DictWriter(f, headers)
    f_csv.writeheader()
    f_csv.writerows(rows)

with open('stocks.csv', 'r') as file:
    csvread = csv.DictReader(file, delimiter=',')

    rows = []
    for row in csvread:
        row['NewColumn'] = '1'
        rows.append(row)

    with open('out.csv', 'w') as out:
        headertowrite = ['Time', 'Symbol', 'NewColumn']
        writer = csv.DictWriter(out, headertowrite, extrasaction='ignore')
        writer.writeheader()
        writer.writerows(rows)

或

--> 在将输出文件的 with 语句中进行修改（读取一行，修改并写入。

import csv
headers = ['Symbol', 'Price', 'Date', 'Time', 'Change', 'Volume']
rows = [{'Symbol':'AA', 'Price':39.48, 'Date':'6/11/2007',
         'Time':'9:36am', 'Change':-0.18, 'Volume':181800},
        {'Symbol':'AIG', 'Price': 71.38, 'Date':'6/11/2007',
         'Time':'9:36am', 'Change':-0.15, 'Volume': 195500},
        {'Symbol':'AXP', 'Price': 62.58, 'Date':'6/11/2007',
         'Time':'9:36am', 'Change':-0.46, 'Volume': 935000}]

with open('stocks.csv','w') as f:
    f_csv = csv.DictWriter(f, headers)
    f_csv.writeheader()
    f_csv.writerows(rows)

with open('stocks.csv', 'r') as file:
    csvread = csv.DictReader(file, delimiter=',')

    with open('out.csv', 'w') as out:
        headertowrite = ['Time', 'Symbol', 'NewColumn']
        writer = csv.DictWriter(out, headertowrite, extrasaction='ignore')
        writer.writeheader()
        for row in csvread:
            row['NewColumn'] = '1'
            writer.writerow(row)

保重！在第二个解决方案中是writerow 而不是writerows！

请注意，我还强烈推荐第二种解决方案，它在内存消耗方面更具可扩展性。