最近在做數(shù)據(jù)分析的練習(xí)時(shí)，對(duì)一段python代碼中的for循環(huán)不是很理解，代碼如下：print('修正數(shù)據(jù)類型（數(shù)據(jù)清理）') import?unicodecsv from?datetime?import?datetime?as?dt def?read_csv(filename): ????with?open(filename,?'rb')?as?f:?? ????????reader?=?unicodecsv.DictReader(f) ????????return?list(reader)????? def?parse_date(date):?????#將字符串的date轉(zhuǎn)換成python的日期時(shí)間對(duì)象 ????if?date?==?'':????????#檢查日期是否為空字符串 ????????return?None ????else: ????????return?dt.strptime(date,?'%Y-%m-%d')?#將字符串解析成日期 daily_engagement?=?read_csv('daily-engagement.csv') print(daily_engagement[0]) #?清理?engagement?的數(shù)據(jù)類型 for?engagement_record?in?daily_engagement: ????engagement_record['utc_date']?=?parse_date(engagement_record['utc_date']) print('\n每日參與學(xué)生數(shù)據(jù)：') print(daily_engagement[0])結(jié)果輸出如下：修正數(shù)據(jù)類型（數(shù)據(jù)清理）OrderedDict([('acct', '0'), ('utc_date', '2015-01-09'), ('num_courses_visited', '1.0'), ('total_minutes_visited', '11.6793745'), ('lessons_completed', '0.0'), ('projects_completed', '0.0')])每日參與學(xué)生數(shù)據(jù)：OrderedDict([('acct', '0'), ('utc_date', datetime.datetime(2015, 1, 9, 0, 0)), ('num_courses_visited', '1.0'), ('total_minutes_visited', '11.6793745'), ('lessons_completed', '0.0'), ('projects_completed', '0.0')])在讀取了daily-engagement.csv文件后，打印變量daily_engagement[0]時(shí)，('utc_date', '2015-01-09')中的日期還只是字符串我的問題是，在對(duì)變量daily_engagement進(jìn)行遍歷后，為什么在打印同樣的daily_engagement時(shí)，里面的utc_date就變成了datetime.datetime(2015, 1, 9, 0, 0)？我在看這個(gè)for循環(huán)的時(shí)候只看出了對(duì)變量engagement_record作了修改，看不出有對(duì)daily_engagement作賦值之類的操作，但卻能夠打印修改后的結(jié)果請(qǐng)大神指教個(gè)中原理，謝謝！