What is the difference between a shallow copy and a deep copy of a nested list with mutable elements in Python?
Davide S

In Python, when dealing with nested lists that contain mutable elements (such as lists or other mutable objects), understanding the difference between a shallow copy and a deep copy is important. Both copying methods create a new list, but they differ in how they handle the nested objects. Here's a long-form explanation of the distinctions: Shallow Copy: 1. Purpose: - A shallow copy creates a new list object but retains references to the nested objects within the original list. - It allows multiple lists to share the same nested objects, resulting in a shallow relationship. 2. Copying Behavior: - When a shallow copy is performed on a nested list, a new list object is created, and the top-level elements (or references) of the original list are copied to the new list. - However, the nested objects themselves are not duplicated. Instead, references to those objects are stored in the new list. - This means that modifications made to the nested objects in the shallow copy will affect the original list and vice versa. 3. Creating a Shallow Copy: - To create a shallow copy, you can use the slicing syntax or thecopy() method from thecopy module. - Example:

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import copy

original_list = [[1, 2, 3], [4, 5, 6]]

# Using slicing:
shallow_copy = original_list[:]

# Using copy():
shallow_copy = copy.copy(original_list)

Deep Copy: 1. Purpose: - A deep copy creates a completely independent copy of the original list, including all nested objects. - It creates a new list with its own copies of the nested objects, resulting in a deep and independent relationship. 2. Copying Behavior: - When a deep copy is performed on a nested list, a new list object is created, and new copies of the nested objects are recursively created and stored in the new list. - This ensures that modifications made to the nested objects in the deep copy do not affect the original list and vice versa. 3. Creating a Deep Copy: - To create a deep copy, you can use thedeepcopy() method from thecopy module. - Example:

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import copy

original_list = [[1, 2, 3], [4, 5, 6]]

deep_copy = copy.deepcopy(original_list)

To summarize: - A shallow copy creates a new list but retains references to the nested objects, resulting in a shared relationship between the original list and the copy. - A deep copy creates an independent copy of the original list and all its nested objects, resulting in a deep and independent relationship. Consider the following code snippet to understand the difference:

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import copy

original_list = [[1, 2, 3], [4, 5, 6]]

# Modifying the nested list in the shallow copy affects the original list
shallow_copy = copy.copy(original_list)
shallow_copy[0].append(4)
print(original_list)  # Output: [[1, 2, 3, 4], [4, 5, 6]]

# Modifying the nested list in the deep copy does not affect the original list
deep_copy = copy.deepcopy(original_list)
deep_copy[0].append(5)
print(original_list)  # Output: [[1, 2, 3, 4], [4, 5, 6]]

In the example, modifying the nested list within the shallow copy affects both the shallow copy and the original list since they reference the same object. However, modifying the nested list within the deep copy has no impact on the original list, demonstrating the independent nature of the deep copy. It's important to choose the appropriate copying method based on your specific requirements and the level of independence you need between the original list and the copied list.

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