Nested list won't copy without assignment in Python

Asked Mar 07 '23 at 22:49

Active Mar 07 '23 at 22:55

Viewed 23 times

I want to copy a list and make changes to the copy without affecting the original list.

a = [[2,3],[4,5]]
b = list(a)

print(b)

del b[0][0]

print(b)
print(a)

I was expecting a = [[2, 3], [4, 5]] in the end.

However, the output was

[[2, 3], [4, 5]]
[[3], [4, 5]]
[[3], [4, 5]]

How can I fix this?

edited Mar 07 '23 at 22:51

Michael M.

asked Mar 07 '23 at 22:49

2

You need a nested copy e.g. `b = [l[:] for l in a]` – Nick Mar 07 '23 at 22:53
1

A deep copy is best: `import copy; b = copy.deepcopy(a)` – Lover of Structure Mar 07 '23 at 23:08
You are making changes not to the list you copied, but to the list *inside* the list you copied, and since you didn't copy those elements (i.e. `list(a)` creates a *shallow* copy), then of course, those changes are reflected in your copy. – juanpa.arrivillaga Mar 07 '23 at 23:14

0 Answers0