Python 使用嵌套列表传递值？

Question

我遇到了一个令我沮丧的简单问题。我确信我犯了一个非常明显的错误，但我似乎找不到它。我希望 l_old 在进行迭代之前成为 l 的副本。我有以下 Python 脚本。

import random
def runtest(n,steps):
    l=[]
    for i in range(n):
        sublist=[]
        for j in range(n):
            sublist.append(0)
        l.append(sublist)


    for k in range(n-1):
        i = random.randint(0,n-1)
        j = random.randint(0,n-1)
        while(l[i][j]==1):
            i = random.randint(0,n-1)
            j = random.randint(0,n-1)
        l[i][j]=1
    l_old = list(l)
    print("L_OLD BEFORE ITERATION", l_old)
    for k in range(steps):
        for i in range(n):
            for j in range(n):
                num = 0
                if i is not 0:
                    num+=l[i-1][j]
                if i is not n-1:
                    num+=l[i+1][j]
                if j is not 0:
                    num+=l[i][j-1]
                if j is not n-1:
                    num+=l[i][j+1]
                if(num > 1):
                    l[i][j]=1
    print("L_OLD AFTER ITERATION",l_old)
    return sum([sum(item) for item in l]), l_old, l

但是，我的输出将如下所示:

BEFORE ITERATION [[0, 0, 0, 0, 0], [0, 0, 1, 0, 0], [1, 0, 1, 0, 0], [0, 0, 0, 0, 0], [1, 0, 0, 0, 0]]
AFTER ITERATION: [[0, 0, 0, 0, 0], [1, 1, 1, 0, 0], [1, 1, 1, 0, 0], [1, 1, 1, 0, 0], [1, 1, 1, 0, 0]]

所以即使我复制了 l 的值而不是对 l 的引用，它仍然在改变。我尝试了 l[:] 得到了相同的结果。我缺少什么？我认为这与它是一个嵌套列表这一事实有关，但是我该如何编写它以便所有嵌套列表都按值复制？

Answer 1

I think it has to do with the fact that it's a nested list,

是的。您已经完成了浅复制:

l_old = list(l)

使用l[:]进行切片也是浅拷贝。

but how can I write it such that all nested lists are copied by value?

尝试深层复制:

from copy import deepcopy
l_old = deepcopy(l)