Python Shallow Copy and Deep Copy

In this article, you’ll learn about shallow copy and deep copy in Python with the help of examples.

Copy an Object in Python

In Python, we use = operator to create a copy of an object. You may think that this creates a new object; it doesn't. It only creates a new variable that shares the reference of the original object.

Let's take an example where we create a list named old_list and pass an object reference to new_list using = operator.

Example 1: Copy using = operator

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)
print('ID of Old List:', id(old_list))

print('New List:', new_list)
print('ID of New List:', id(new_list))

When we run above program, the output will be:

Old List: [[1, 2, 3], [4, 5, 6], [7, 8, 9]]
ID of Old List: 140673303268168

New List: [[1, 2, 3], [4, 5, 6], [7, 8, 9]]
ID of New List: 140673303268168

As you can see from the output both variables old_list and new_list shares the same id i.e 140673303268168.

So, if you want to modify any values in new_list or old_list, the change is visible in both.


Essentially, sometimes you may want to have the original values unchanged and only modify the new values or vice versa. In Python, there are two ways to create copies:

  1. Shallow Copy
  2. Deep Copy

To make these copy work, we use the copy module.


Copy Module

We use the copy module of Python for shallow and deep copy operations. Suppose, you need to copy the compound list say x. For example:

import copy
copy.copy(x)
copy.deepcopy(x)

Here, the copy() return a shallow copy of x. Similarly, deepcopy() return a deep copy of x.


Shallow Copy

A shallow copy creates a new object which stores the reference of the original elements.

So, a shallow copy doesn't create a copy of nested objects, instead it just copies the reference of nested objects. This means, a copy process does not recurse or create copies of nested objects itself.

Example 2: Create a copy using shallow copy

import copy

old_list = [[1, 2, 3], [4, 5, 6], [7, 8, 9]]
new_list = copy.copy(old_list)

print("Old list:", old_list)
print("New list:", new_list)

When we run the program , the output will be:

Old list: [[1, 2, 3], [4, 5, 6], [7, 8, 9]]
New list: [[1, 2, 3], [4, 5, 6], [7, 8, 9]]

In above program, we created a nested list and then shallow copy it using copy() method.

This means it will create new and independent object with same content. To verify this, we print the both old_list and new_list.

To confirm that new_list is different from old_list, we try to add new nested object to original and check it.


Example 3: Adding [4, 4, 4] to old_list, using shallow copy

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)
print("New list:", new_list)

When we run the program, it will output:

Old list: [[1, 1, 1], [2, 2, 2], [3, 3, 3], [4, 4, 4]]
New list: [[1, 1, 1], [2, 2, 2], [3, 3, 3]]

In the above program, we created a shallow copy of old_list. The new_list contains references to original nested objects stored in old_list. Then we add the new list i.e [4, 4, 4] into old_list. This new sublist was not copied in new_list.

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


Example 4: Adding new nested object using Shallow copy

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)
print("New list:", new_list)

When we run the program, it will output:

Old list: [[1, 1, 1], [2, 'AA', 2], [3, 3, 3]]
New list: [[1, 1, 1], [2, 'AA', 2], [3, 3, 3]]

In the above program, we made changes to old_list i.e old_list[1][1] = 'AA'. Both sublists of old_list and new_list at index [1][1] were modified. This is because, both lists share the reference of same nested objects.


Deep Copy

A deep copy creates a new object and recursively adds the copies of nested objects present in the original elements.

Let’s continue with example 2. However, we are going to create deep copy using deepcopy() function present in copy module. The deep copy creates independent copy of original object and all its nested objects.

Example 5: Copying a list using deepcopy()

import copy

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

print("Old list:", old_list)
print("New list:", new_list)

When we run the program, it will output:

Old list: [[1, 1, 1], [2, 2, 2], [3, 3, 3]]
New list: [[1, 1, 1], [2, 2, 2], [3, 3, 3]]

In the above program, we use deepcopy() function to create copy which looks similar.

However, if you make changes to any nested objects in original object old_list, you’ll see no changes to the copy new_list.


Example 6: Adding a new nested object in the list using Deep copy

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)
print("New list:", new_list)

When we run the program, it will output:

Old list: [[1, 1, 1], ['BB', 2, 2], [3, 3, 3]]
New list: [[1, 1, 1], [2, 2, 2], [3, 3, 3]]

In the above program, when we assign a new value to old_list, we can see only the old_list is modified. This means, both the old_list and the new_list are independent. This is because the old_list was recursively copied, which is true for all its nested objects.