Why first items are removed from both lists?

Question

list1 = [
    [0,1,2,3,4],
    [0,1,2,3,4]
]
list2 = list1.copy()
for i in range(len(list2)):
    list2[i].pop(0)
print(list1)#[[1, 2, 3, 4], [1, 2, 3, 4]]
print(list2)#[[1, 2, 3, 4], [1, 2, 3, 4]]

Copying list1 with copy() method in list2 and removing items from nested lists in list2 it shouldn't remove those items in list1. Why it happens?????

You need a deep copy. `from copy import deepcopy` then `list2 = deepcopy(list1)` — Unmitigated, Mar 26 '23 at 20:00
Does this answer your question? [How to deep copy a list?](https://stackoverflow.com/questions/17873384/how-to-deep-copy-a-list) — Fractalism, Mar 26 '23 at 20:05

score 1 · Answer 1 · answered Mar 26 '23 at 20:03

This occurs because of the difference between a shallow copy vs a deep copy. copy() creates a shallow copy vs deepcopy() creates a deep copy. Here's what the documentation has to say:

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.

orange river · Answer 2 · 2023-03-26T20:06:08.207

0

Because the copy() operation is not independent,you could use look at the deepcopy().If you want a nomarl copy operation, you could use deepcopy() to do that.

edited Mar 26 '23 at 20:06

answered Mar 26 '23 at 20:03

orange river

13
4

score 0 · Answer 3 · answered Mar 26 '23 at 20:16

complementing the before answers, you can change to this:

import copy

list1: list[list[int]] = [
    [0, 1, 2, 3, 4],
    [0, 1, 2, 3, 4],
]
list2 = copy.deepcopy(list1)
for pos, items in enumerate(list2):
    list2[pos].pop(0)
print(list1)  # [[0, 1, 2, 3, 4], [0, 1, 2, 3, 4]]
print(list2)  # [[1, 2, 3, 4], [1, 2, 3, 4]]

Why first items are removed from both lists?

3 Answers3