Why does that both lists are modified?

Asked Jan 13 '22 at 22:09

Active Jan 13 '22 at 22:46

Viewed 50 times

So I'm trying to create a back up of a list but when I want to modify the backup variable, both are being modified.

a = [[1, 2], [1, 2]]
b = a
del a[-1]

print(a)
print(b)

Output:

[[1, 2]]
[[1, 2]]

What can I do so my output looks like this:

[[1, 2],[1, 2]
[[1, 2]

edited Jan 13 '22 at 22:46

martineau

asked Jan 13 '22 at 22:09

ZBNomek

5

There is only one list, with two references to it. – Fred Larson Jan 13 '22 at 22:10
`b = a.copy()`. – Tim Roberts Jan 13 '22 at 22:12
For this use case `copy` is enough. But you need [`deepcopy`](https://docs.python.org/3/library/copy.html#copy.deepcopy) if you do something like `a[0][0] = 42`. – Matthias Jan 13 '22 at 22:14
For reference: [Facts and myths about Python names and values](https://nedbatchelder.com/text/names.html). – martineau Jan 13 '22 at 22:48

0 Answers0