How to deep copy a list? – python

In Python, to create a deep copy of a list (or any mutable object), you can use the copy module or other techniques. A deep copy means that all objects contained in the original list are recursively copied, rather than just copying the references to those objects.

Method 1: Using copy Module

Python provides a deepcopy() function in the copy module for creating a deep copy of a list.

import copy

original_list = [1, [2, 3], [4, 5]]
deep_copy_list = copy.deepcopy(original_list)

# Modify the deep copy
deep_copy_list[1][0] = 99

print("Original List:", original_list)  # Output: [1, [2, 3], [4, 5]]
print("Deep Copy List:", deep_copy_list)  # Output: [1, [99, 3], [4, 5]]

Explanation:

• copy.deepcopy() creates a new list and recursively copies all the objects, meaning changes to deep_copy_list will not affect original_list.
• In the example, modifying the deep copy does not affect the original list.

Method 2: Using List Comprehension for Simple Lists

If the list does not contain nested objects (e.g., no sublists), you can simply copy the list using a list comprehension.

original_list = [1, 2, 3, 4]
deep_copy_list = [item for item in original_list]

# Modify the deep copy
deep_copy_list[0] = 99

print("Original List:", original_list)  # Output: [1, 2, 3, 4]
print("Deep Copy List:", deep_copy_list)  # Output: [99, 2, 3, 4]

Method 3: Using list() Constructor

Another way to create a shallow copy (not deep, but good for non-nested lists) is using the list() constructor:

original_list = [1, 2, 3, 4]
deep_copy_list = list(original_list)

# Modify the deep copy
deep_copy_list[0] = 99

print("Original List:", original_list)  # Output: [1, 2, 3, 4]
print("Deep Copy List:", deep_copy_list)  # Output: [99, 2, 3, 4]

Method 4: Using Slicing for Non-Nested Lists

For simple, non-nested lists, you can also use slicing to create a shallow copy:

original_list = [1, 2, 3, 4]
deep_copy_list = original_list[:]

# Modify the deep copy
deep_copy_list[0] = 99

print("Original List:", original_list)  # Output: [1, 2, 3, 4]
print("Deep Copy List:", deep_copy_list)  # Output: [99, 2, 3, 4]

Conclusion:

• Use copy.deepcopy() when working with nested objects or lists, as it ensures all levels of the list are copied.
• For non-nested lists, you can use list comprehension, list(), or slicing to copy the list, but these are shallow copies, meaning they will not fully copy nested objects.