Copying Lists in Python

When working with lists in Python, it’s essential to understand how copying lists works, as it can impact your program’s behavior. In this article, we’ll explore the concepts of mutability, shallow copy, and deep copy in the context of Python lists, and provide examples of when and how to use each technique.

Mutability in Python Lists

Before diving into copying lists, it’s crucial to grasp the concept of mutability. In Python, lists are mutable, which means their contents can be changed after creation. When you assign a list to a new variable, you’re creating a reference to the same list in memory. This can lead to unexpected behaviour if you’re not careful when working with copies.

Shallow Copy: Copying the Outer Structure

A shallow copy of a list creates a new list object but only copies the outer structure of the original list. The inner elements, or objects within the list, remain references to the same objects as the original list. You can create a shallow copy using various techniques, such as slicing, the copy() method, or the list() constructor.

Slicing is a really handy and powerful way to work with lists in Python. You can also use the copy() method when you’re working directly with list objects and want a concise way to create a shallow copy. Use the list() constructor when you need to create a shallow copy of any iterable, not just lists, and want to convert it into a list.

original_list = [1, [2, 3], 4]
shallow_copy = original_list[:]  # Using slicing

original_list = [1, [2, 3], 4]
shallow_copy = original_list.copy()  # Using the copy() method

original_list = [1, [2, 3], 4]
shallow_copy = list(original_list)  # Using the list() constructor

In these examples, shallow_copy is a new list, but it shares references with original_list for the inner elements. If you modify an inner element in one list, it will affect the other:

shallow_copy[1][0] = 99
print(original_list)  # [1, [99, 3], 4]

When to Use Shallow Copy

Shallow copies are useful when you want to duplicate the structure of a list but don’t need to create entirely independent copies of the objects within it. However, be cautious when working with mutable objects within the list, as changes will propagate to both the original and shallow copy.

Deep Copy: Creating Independent Copies

A deep copy, on the other hand, creates entirely independent copies of both the outer structure and the inner objects of a list. You can create a deep copy using the copy module’s deepcopy() function.

import copy

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

With a deep copy, changes to one list won’t affect the other:

deep_copy[1][0] = 99
print(original_list)  # [1, [2, 3], 4]

When to Use Deep Copy

Deep copies are necessary when you need to work with lists containing mutable objects and want to ensure that changes in one list do not impact another. However, keep in mind that deep copying can be slower and consume more memory than shallow copying.

Example of Avoiding Bugs by Choosing the Correct Python List Copy Method

Let’s consider a scenario where a shallow copy creates a bug and demonstrate how deep copying can solve the problem.

Scenario: Shopping Cart Items

Imagine you’re developing a shopping cart application, and you want to keep track of items in the cart using a list. Each item is represented as a dictionary with ‘name’ and ‘quantity’ keys. Here’s your initial code:

# Initial Shopping Cart
shopping_cart = [{'name': 'apple', 'quantity': 3}, {'name': 'banana', 'quantity': 2}]

# Shallow Copy
cart_copy = shopping_cart.copy()

Bug with Shallow Copy:

Now, suppose you want to add more apples to your cart, so you update the quantity in cart_copy. However, this change unexpectedly affects your original cart as well:

# Modify the copy
cart_copy[0]['quantity'] = 5

# Output: [{'name': 'apple', 'quantity': 5}, {'name': 'banana', 'quantity': 2}]

Here, the shallow copy shared the same dictionaries inside both lists. Modifying one affected the other, which is not the behavior you intended.

Solution with Deep Copy:

To avoid this issue and ensure that changes in one cart do not affect the other, you can use deep copying:

import copy

# Deep Copy
cart_copy_deep = copy.deepcopy(shopping_cart)

Now, when you modify cart_copy_deep, the original shopping_cart remains unchanged:

# Modify the deep copy
cart_copy_deep[0]['quantity'] = 5

# Output: [{'name': 'apple', 'quantity': 3}, {'name': 'banana', 'quantity': 2}]

By using deep copying, you prevent unintended side effects and maintain data integrity, making it a suitable solution for scenarios involving nested mutable objects like dictionaries in this shopping cart example.

Summary: Cases Where Each Copying Technique is Needed

Shallow Copy: Use shallow copies when you want to replicate the structure of a list, but you don’t need to create entirely independent copies of the objects within it. This is suitable for simple lists without mutable objects.

# Shallow copy for simple list
shallow_copy = original_list[:]

Deep Copy: Utilize deep copies when you need to work with lists containing mutable objects, and you want to ensure that changes in one list do not affect another. This is crucial for maintaining data integrity in more complex data structures.

# Deep copy for lists with mutable objects
deep_copy = copy.deepcopy(original_list)


In conclusion, understanding mutability and the differences between shallow and deep copies is essential when working with Python lists. Choosing the appropriate copy technique ensures that your program behaves as expected and helps prevent unexpected side effects when modifying data structures.

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