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?

Answer 1

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.