Python Pandas 单元问题更新时间

Question

疯狂尝试更新数据框中的一列时间条目。我正在打开一个 csv 文件，其中有一列 UTC 时间条目。我可以把这些时间转换成阿拉斯加标准时间，然后打印出新的时间就好了。但是，当我尝试将时间放回数据帧时，虽然我没有收到任何错误，但我也没有在数据帧中获得新时间。保留旧的 UTC 时间。代码在下面，我很好奇我错过了什么。时间有什么特别之处吗？

import glob
import os
import pandas as pd
from datetime import datetime
from statistics import mean

def main():
    AKST = 9
    allDirectories = os.listdir('c:\MyDir\')
    for directory in allDirectories:
        curDirectory = directory.capitalize()
        print('Gathering data from: ' + curDirectory)
        dirPath = 'c:\MyDir\' + directory + '\*.csv'
        # Files are named by date, so sorting by name gives us a proper date order
        files = sorted(glob.glob(dirPath))
        df = pd.DataFrame()
        for i in range(0,len(files)):
            data = pd.read_csv(files[i], usecols=['UTCDateTime', 'current_humidity', 'pm2_5_cf_1', 'pm2_5_cf_1_b'])
            dfTemp = pd.DataFrame(data) # Temporary dataframe to hold our new info
            df = pd.concat([df, dfTemp], axis=0) # Add new info to end of dataFrame
        print("Converting UTC to AKST, this may take a moment.")
        for index, row in df.iterrows():
            convertedDT = datetime.strptime(row['UTCDateTime'], '%Y/%m/%dT%H:%M:%Sz') - pd.DateOffset(hours=AKST)
            print("UTC: " + row['UTCDateTime'])
            df.at[index,'UTCDateTime'] = convertedDT
            print("AKST: " + str(convertedDT))
            print("row['UTCDateTime] = " + row['UTCDateTime'] + '\n') # Should be updated with AKST, but is not!

编辑 - 或者：有没有办法将首次读入数据框的日期转换为日期？看起来这比有两个 for 循环要快。

Answer 1

从您的代码来看，数据似乎已在数据框中正确更新，但您正在打印未更新的行，因为它是从数据框中获取的在更新之前！

 #You are updating df
 df.at[index,'UTCDateTime'] = convertedDT #You are updating df
 # below you are printing row
 print("row['UTCDateTime] = " + row['UTCDateTime']

请参阅下面的示例代码及其输出以获取解释。

data=pd.DataFrame({'Year':  [1982,1983], 'Statut':['Yes',  'No']})
for index, row in data.iterrows():
    data.at[index, 'Year'] = '5000' + str(index)
    print('Printing row which is unchanged : ', row['Year'])
print('Updated Dataframe\n',data)

输出

Printing row which is unchanged :  1982
Printing row which is unchanged :  1983
Updated Dataframe
     Year Statut
0  50000    Yes
1  50001     No

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