When appended a copy of list into another list, after altering the original list it also gets altered in another list

Question

k=[
[7, 1, 3, 6, 8, 5, 5, 6, 4],
[7, 2, 6, 2, 2, 8, 3, 9, 6],
[3, 3, 8, 6, 1, 3, 4, 5, 9],
[4, 5, 9, 8, 6, 6, 1, 3, 4],
[2, 8, 1, 4, 8, 6, 9, 5, 1],
[4, 7, 8, 6, 1, 8, 5, 8, 4],
[6, 7, 6, 4, 8, 6, 6, 7, 2],
[9, 8, 6, 3, 8, 8, 5, 5, 9],
[9, 5, 7, 5, 1, 1, 8, 6, 5]
]
a=[]
c=0

def foo():
    global a
    global k
    global c
    a.append(k.copy())
    print(a)
    for i in range(9):
        for j in range(9):
            k[i][j]=1
    print(a)

foo()

Expected Output:

[[[7, 1, 3, 6, 8, 5, 5, 6, 4], [7, 2, 6, 2, 2, 8, 3, 9, 6], [3, 3, 8, 6, 1, 3, 4, 5, 9], [4, 5, 9, 8, 6, 6, 1, 3, 4], [2, 8, 1, 4, 8, 6, 9, 5, 1], [4, 7, 8, 6, 1, 8, 5, 8, 4], [6, 7, 6, 4, 8, 6, 6, 7, 2], [9, 8, 6, 3, 8, 8, 5, 5, 9], [9, 5, 7, 5, 1, 1, 8, 6, 5]]]

Generated Output:

[[[7, 1, 3, 6, 8, 5, 5, 6, 4], [7, 2, 6, 2, 2, 8, 3, 9, 6], [3, 3, 8, 6, 1, 3, 4, 5, 9], [4, 5, 9, 8, 6, 6, 1, 3, 4], [2, 8, 1, 4, 8, 6, 9, 5, 1], [4, 7, 8, 6, 1, 8, 5, 8, 4], [6, 7, 6, 4, 8, 6, 6, 7, 2], [9, 8, 6, 3, 8, 8, 5, 5, 9], [9, 5, 7, 5, 1, 1, 8, 6, 5]]]

[[[1, 1, 1, 1, 1, 1, 1, 1, 1], [1, 1, 1, 1, 1, 1, 1, 1, 1], [1, 1, 1, 1, 1, 1, 1, 1, 1], [1, 1, 1, 1, 1, 1, 1, 1, 1], [1, 1, 1, 1, 1, 1, 1, 1, 1], [1, 1, 1, 1, 1, 1, 1, 1, 1], [1, 1, 1, 1, 1, 1, 1, 1, 1], [1, 1, 1, 1, 1, 1, 1, 1, 1], [1, 1, 1, 1, 1, 1, 1, 1, 1]]]

Also check [What is the difference between a deep copy and a shallow copy?](https://stackoverflow.com/q/184710/4046632) — buran, Sep 23 '21 at 09:18

score 0 · Answer 1 · answered Sep 23 '21 at 09:19

.copy method of list is shallow, you need to use copy.deepcopy if you want to get totally independent copy, consider following example

import copy
k1 = [[1,2],[3,4]]
k2 = k1.copy()
k3 = copy.deepcopy(k1)
k1[0][0] = 0
print(k2)
print(k3)

output

[[0, 2], [3, 4]]
[[1, 2], [3, 4]]

Using .copy is fine if you are working with flat list of immutable objects, you have list of list which are mutable.

score 0 · Accepted Answer · answered Sep 23 '21 at 09:32

With copy() you perform a shallow copy. You need a deep copy. See Docs for more information

The difference between shallow and deep copying is only relevant for compound objects (objects that contain other objects, like lists or class instances):

A shallow copy constructs a new compound object and then (to the extent possible) inserts references into it to the objects found in the original.

A deep copy constructs a new compound object and then, recursively, inserts copies into it of the objects found in the original.

from copy import deepcopy

a.append(deepcopy(k))
print(a)
for i in range(9):
    for j in range(9):
        k[i][j]=1
print(a)

When appended a copy of list into another list, after altering the original list it also gets altered in another list

2 Answers2