Why is the list.copy() method shallow?

Question

In the official Python documentation said that list.copy() returns a shallow copy of the list. But according to the following code it is deep copy since the change of one list does not lead to the change in another.

>>> num1 = [1,2,3,4,5]
>>> num2 = num1.copy()
>>> num1.append(9)
>>> num1
[1, 2, 3, 4, 5, 9]
>>> num2
[1, 2, 3, 4, 5]

What is the problem? Where is a mistake?

That's what a shallow copy is. You're confusing a shallow copy with not-being-a-copy. — khelwood, Sep 28 '20 at 14:40
The copy is shallow because if `num1` contained lists or dictionaries, those structures would not be copied. — CryptoFool, Sep 28 '20 at 14:41
If copy() did what you are thinking, why would it be a necessary function? Wouldn't `num2 = num1` then do the same thing? — CryptoFool, Sep 28 '20 at 14:44
There is no difference between shallow and deep when the list elements are numbers; there is nothing "below" the numbers. — molbdnilo, Sep 28 '20 at 14:46
Can anyone give a link on resource where it explained in details? — Nika Mazulina, Sep 28 '20 at 15:07
@molbdnilo well, in this case, it doesn't matter what the elements are, the behavior would be the same — juanpa.arrivillaga, Sep 28 '20 at 15:09
I still do not understand the difference. Can anyone give link on literature or video with detailed explanation? — Nika Mazulina, Sep 28 '20 at 15:18

score 2 · Answer 1 · answered Sep 28 '20 at 14:42

2

This example will demonstrate why this is a shallow copy

>>> num1 = [[1,2,3],[4,5,6]]
>>> num2 = num1.copy()
>>> num1[0].append(9)
>>> num1
[[1, 2, 3, 9], [4, 5, 6]]
>>> num2
[[1, 2, 3, 9], [4, 5, 6]]

Since the original list contained mutable elements, the outer copy contains shallow copies to the mutable elements.

answered Sep 28 '20 at 14:42

Cory Kramer

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But why it does not work with simple list? I do not understand. Why in case of simple list it does not effect the copied one – Nika Mazulina Sep 28 '20 at 14:49
@NikaMazulina Because it's a copy. – khelwood Sep 28 '20 at 14:50
But in case of nested list for example it effects the copy. It is also just a copy. What is the difference? – Nika Mazulina Sep 28 '20 at 14:52
@NikaMazulina because `num1[0].append(9)` isn't changing the list, it is changing an *element inside the list*. – juanpa.arrivillaga Sep 30 '20 at 09:06

Why is the list.copy() method shallow?

1 Answers1