Python列表复制和切片修改原始列表

Question

我有一个列表如下所示：

voting_records = [['1.', 'DIRECTOR', 'Management'],
 ['1', 'Allison Grant Williams', 'For'],
 ['2', 'Sheila Hartnett-Devlin', 'For'],
 ['3', 'James Jessee', 'For'],
 ['4', 'Teresa Polley', 'For'],
 ['5', 'Ashley T. Rabun', 'For'],
 ['6', 'James E. Ross', 'For'],
 ['7', 'Rory Tobin', 'For']]

我使用列表理解创建了一个名为

voting_records_short

的列表副本：

cnames = ["Record","Proposal","Sponsor","VoteCast"]
voting_records_short = [record for record in voting_records if len(record) < len(cnames)]

然后我创建了一片

voting_records_short

并保存在

directors

中。然后我在 for 循环中修改

directors

列表，如下所示：

if len(voting_records_short) == 0: pass
else:
    ifdirector = ["DIRECTOR" in record for record in voting_records_short]
    if True in ifdirector:
        directorindx = int(ifdirector.index(True))
        directorline = [voting_records_short[directorindx]]
        directors = voting_records_short[directorindx+1:]
        sponsoridx = cnames.index("Sponsor")
        for ls in range(len(directors)):
            directors[ls][0] = directorline[0][0] + directors[ls][0]
            directors[ls][1] = directorline[0][1] + " " +directors[ls][1]
            directors[ls].insert(sponsoridx,directorline[0][-1])

为什么在 for 循环中修改导演列表会同时修改

voting_records_short

列表和

voting_records

，如下面的输出所示：

In [2]: voting_records
Out[2]: 
[['1.', 'DIRECTOR', 'Management'],
 ['1.1', 'DIRECTOR Allison Grant Williams', 'Management', 'For'],
 ['1.2', 'DIRECTOR Sheila Hartnett-Devlin', 'Management', 'For'],
 ['1.3', 'DIRECTOR James Jessee', 'Management', 'For'],
 ['1.4', 'DIRECTOR Teresa Polley', 'Management', 'For'],
 ['1.5', 'DIRECTOR Ashley T. Rabun', 'Management', 'For'],
 ['1.6', 'DIRECTOR James E. Ross', 'Management', 'For'],
 ['1.7', 'DIRECTOR Rory Tobin', 'Management', 'For']]

In [9]: voting_records_short
Out[9]: 
[['1.', 'DIRECTOR', 'Management'],
 ['1.1', 'DIRECTOR Allison Grant Williams', 'Management', 'For'],
 ['1.2', 'DIRECTOR Sheila Hartnett-Devlin', 'Management', 'For'],
 ['1.3', 'DIRECTOR James Jessee', 'Management', 'For'],
 ['1.4', 'DIRECTOR Teresa Polley', 'Management', 'For'],
 ['1.5', 'DIRECTOR Ashley T. Rabun', 'Management', 'For'],
 ['1.6', 'DIRECTOR James E. Ross', 'Management', 'For'],
 ['1.7', 'DIRECTOR Rory Tobin', 'Management', 'For']]

我原以为只有

directors

列表会被修改（也许

voting_records_short

但我不确定）但是为什么原来的

voting_records

列表被修改了？我会很感激任何帮助？

Answer 1

您正在迭代原始的 voting_records 并将对这些记录中的每一个的引用存储在一个新列表中；因此，对带有引用的记录所做的更改将修改两个列表——如上所述：deepcopy

from copy import deepcopy
copy_records = deepcopy(voting_records)
voting_records_short = [record for record in copy_records if len(record) < len(cnames)]

运行代码后：

[['1.', 'DIRECTOR', 'Management'], ['1', 'Allison Grant Williams', 'For'], ['2', 'Sheila Hartnett-Devlin', 'For'], ['3', 'James Jessee', 'For'], ['4', 'Teresa Polley', 'For'], ['5', 'Ashley T. Rabun', 'For'], ['6', 'James E. Ross', 'For'], ['7', 'Rory Tobin', 'For']]
[['1.', 'DIRECTOR', 'Management'], ['1.1', 'DIRECTOR Allison Grant Williams', 'Management', 'For'], ['1.2', 'DIRECTOR Sheila Hartnett-Devlin', 'Management', 'For'], ['1.3', 'DIRECTOR James Jessee', 'Management', 'For'], ['1.4', 'DIRECTOR Teresa Polley', 'Management', 'For'], ['1.5', 'DIRECTOR Ashley T. Rabun', 'Management', 'For'], ['1.6', 'DIRECTOR James E. Ross', 'Management', 'For'], ['1.7', 'DIRECTOR Rory Tobin', 'Management', 'For']]