Issue while modifying elements of nested list in Python

Question

In this code, I was expecting that list A will have the same value even if I assign it to another temp variable and then print the argument A using the function foo. However, if A was a scalar e.g. A=3 then the value of A remains the same even after calling foo.

Where am I going wrong? Is there a problem in the scope of variables? I found some related Strange behavior of lists in python answer but couldn't figure out a fix for my problem.

A = [ [ 0 for i in range(3) ] for j in range(3) ]

def foo(input):

    temp= input
    temp[0][0]=12
    print(input)

print(A)
answer = foo(A)

Output:

[[0, 0, 0], [0, 0, 0], [0, 0, 0]]
[[12, 0, 0], [0, 0, 0], [0, 0, 0]]

@Anwarvic, that outputs `None`. `temp` and `input` point to same object, so if you change one that reflects in the other. Btw, don't name as `input`. — Austin, Mar 03 '19 at 07:52
Your code will fail with `A = 3` on `temp[0][0]`, as `temp` would be `3` by this time. You should have a look at https://nedbatchelder.com/text/names.html. — Thierry Lathuille, Mar 03 '19 at 07:57

score 0 · Accepted Answer · answered Mar 03 '19 at 07:53

0

instead of input, use the .copy this will make a copy of the array and assign new addresses to temp, what you do is shallow copying, where temp = input, just copies the address of the input array to temp, rather than making a copy of the list.

So you may either do foo(A.copy()) or temp=input.copy() also note input is not a good name since it is already assigned to a python function, use something like foo_arg or something

answered Mar 03 '19 at 07:53

Imtinan Azhar

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Thanks! I used "deepcopy" from copy library, it served my purpose. – Shadab Azeem Mar 03 '19 at 08:29

score 0 · Answer 2 · answered Mar 03 '19 at 07:58

0

Please this, I used deepcopy

from copy import copy, deepcopy

A = [ [ 0 for i in range(3) ] for j in range(3) ]

def foo(input):

    temp = deepcopy(input)
    temp[0][0]=12
    return temp

print('origin', A)
answer = foo(A)
print('after', A)
print('result', answer)

Result:

origin [[0, 0, 0], [0, 0, 0], [0, 0, 0]]
after [[0, 0, 0], [0, 0, 0], [0, 0, 0]]
result [[12, 0, 0], [0, 0, 0], [0, 0, 0]]

answered Mar 03 '19 at 07:58

Winner

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Thanks a lot for the help. I also found a good resource over here: https://www.geeksforgeeks.org/copy-python-deep-copy-shallow-copy/ – Shadab Azeem Mar 03 '19 at 08:24

Issue while modifying elements of nested list in Python

2 Answers2