csv DictReader的“正在工作”失败

问题描述：

我在写一个脚本，需要将CSV读取到DictReader，在字段（数据管理）上做一些工作，然后通过DictWriter将DictReader输出到csv。csv DictReader的“正在工作”失败

如果我读了CSV然后写出Dict，过程就起作用了。

#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 - 如果我添加了一个新列，看来我失去了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让我在DictReader执行工作之前，我把它写？

答

由于csvread已完全被您的第一个for循环占用，因此我们的with语句将输出一个空白文件，您可能会注意到。

您应该：

- >保存行到一个变量

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

- >让内你的修改与将输出的文件（阅读声明一行，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)

小心！这是writerow不是writerows在第二个解决方案！

请注意，我还强烈建议第二个解决方案，它在内存消耗方面更具可扩展性。

csv DictReader的“正在工作”失败

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