user2528042 user2528042 - 4 months ago 11
Python Question

Does a slicing operation give me a deep or shallow copy?

The official Python docs say that using the slicing operator and assigning in Python makes a shallow copy of the sliced list.

But when I write code for example:

o = [1, 2, 4, 5]
p = o[:]


And when I write:

id(o)
id(p)


I get different id's and also appending one one list does not reflect in the other list. Isn't it creating a deep copy or is there somewhere I am going wrong?

Answer

You are creating a shallow copy, because nested values are not copied, merely referenced. A deep copy would create copies of the values referenced by the list too.

Demo:

>>> lst = [{}]
>>> lst_copy = lst[:]
>>> lst_copy[0]['foo'] = 'bar'
>>> lst_copy.append(42)
>>> lst
[{'foo': 'bar'}]
>>> id(lst) == id(lst_copy)
False
>>> id(lst[0]) == id(lst_copy[0])
True

Here the nested dictionary is not copied; it is merely referenced by both lists. The new element 42 is not shared.

Remember that everything in Python is an object, and names and list elements are merely references to those objects. A copy of a list creates a new outer list, but the new list merely receives references to the exact same objects.

A proper deep copy creates new copies of each and every object contained in the list, recursively:

>>> from copy import deepcopy
>>> lst_deepcopy = deepcopy(lst)
>>> id(lst_deepcopy[0]) == id(lst[0])
False
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