Python 列表复制和切片修改原始列表

问：

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']]

我创建了这个列表的副本，名为使用列表推导：voting_records_short

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

然后我创建了一个切片并将其保存在 .然后，我在for循环中修改列表，如下所示：voting_records_shortdirectorsdirectors

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 directors 列表会同时修改列表，如下面的输出所示：voting_records_shortvoting_records

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']]

我原以为只会修改列表（也许，但我不确定），但为什么要修改原始列表？我很感激任何帮助？directorsvoting_records_shortvoting_records