Clone a list in Python

The Problem

After I attempt to copy a list by setting new_list = old_list, any modifications to new_list also affect old_list. What is the proper way to clone a list and avoid this?

The Solution

The code new_list = old_list does not create a second list, but merely causes new_list to reference the same list object as old_list. To make new_list an independent copy of old_list, we must explicitly copy the list. We can do this in two ways:

• Shallow copy: Creates a new list, but does not create copies of the objects that the original list references. Both the original and the cloned list will still reference the same objects.
• Deep copy: Creates a new list and creates copies of every object in the original list references, recursively, ensuring that the two lists are entirely independent.

Performing a shallow copy is sufficient for lists containing simple, immutable types, such as numbers and strings. If a list contains other lists, dictionaries, or custom objects, a deep copy will be required.

We can use one of three methods to make a shallow copy:

1. list.copy: The list.copy method makes a shallow copy of the original list:

   old_list = [1, 2, 3, 4, 5]
   new_list = old_list.copy()
   

2. List slicing: We can also create a clone of a list using list slicing:

   new_list = old_list[:]
   

   This creates a new list by slicing the old list from its first to its final element. It is identical to list.copy but may be less clear.

3. Using the list constructor: We can also create a new list from the existing list by passing it to the list constructor.

   new_list = list(old_list)
   

   Again, this syntax produces identical results to the copy method but may be less clear.

To make a deep copy of the list, we must use the deepcopy function from Python’s built-in copy module:

import copy

old_list = [[1, 2], [3, 4]]
new_list = copy.deepcopy(old_list)

new_list[0][1] = 5 # change inner list in copy

print(old_list) # will print [[1, 2], [3, 4]]
print(new_list) # will print [[1, 5], [3, 4]]