Copy

Fatma Kahveci

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Copy an Object in Python

= does not create a new object. It only creates a new variable that shares the reference of the original object.

old_list = [[1, 2, 3], [4, 5, 6], [7, 8, 'a']]
new_list = old_list

new_list[2][2] = 9

print('Old List:', old_list) # Old List: [[1, 2, 3], [4, 5, 6], [7, 8, 9]]
print('ID of Old List:', id(old_list)) # ID of Old List: 140673303268168

print('New List:', new_list) # [[1, 2, 3], [4, 5, 6], [7, 8, 9]]
print('ID of New List:', id(new_list)) # ID of New List: 140673303268168

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deepcopy

import copy

copy.deepcopy(<object>) returns a deep copy of the object.
The old_list and new_list are independent.

import copy

old_list = [[1, 1, 1], [2, 2, 2], [3, 3, 3]]
new_list = copy.deepcopy(old_list)

old_list[1][0] = 'BB'

print("Old list:", old_list) # Old list: [[1, 1, 1], ['BB', 2, 2], [3, 3, 3]]
print("New list:", new_list) # New list: [[1, 1, 1], [2, 2, 2], [3, 3, 3]]

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shallowcopy

import copy

copy.copy(<object>) returns a shallow copy of the object.
It creates a new object which stores the reference of the original elements.
It doesn’t create a copy of nested objects, instead, it just copies the reference of nested objects.
The new_list contains references to the original nested objects stored in the old_list. Then we add the new list i.e [4, 4, 4] into the old_list. This new sublist was not copied in the new_list.

import copy

old_list = [[1, 1, 1], [2, 2, 2], [3, 3, 3]]
new_list = copy.copy(old_list)

old_list.append([4, 4, 4])

print("Old list:", old_list) # Old list: [[1, 1, 1], [2, 2, 2], [3, 3, 3], [4, 4, 4]]
print("New list:", new_list) # New list: [[1, 1, 1], [2, 2, 2], [3, 3, 3]]

When you change any nested objects in old_list, the changes appear in new_list.

import copy

old_list = [[1, 1, 1], [2, 2, 2], [3, 3, 3]]
new_list = copy.copy(old_list)

old_list[1][1] = 'AA'

print("Old list:", old_list) # Old list: [[1, 1, 1], [2, 'AA', 2], [3, 3, 3]]
print("New list:", new_list) # New list: [[1, 1, 1], [2, 'AA', 2], [3, 3, 3]]

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