Python List copy() Method Tutorial (With Examples)

Introduction to the `copy()` Method

In Python, lists are versatile data structures that allow you to store and manipulate collections of items. The list data type comes with a variety of built-in methods that make it easier to work with lists effectively. One such method is copy(), which is used to create a shallow copy of an existing list.

A shallow copy of a list is a new list containing references to the same objects as the original list. In other words, the new list points to the same elements as the original list, rather than creating copies of those elements themselves. This can have implications when you modify the elements within the copied list. Understanding the copy() method is crucial to avoid unexpected behavior when working with lists.

In this tutorial, we will cover the following topics:

How to use the copy() method to create a shallow copy of a list.
Understanding the difference between a shallow copy and a deep copy.
Examples demonstrating the usage of the copy() method in various scenarios.

Creating a Shallow Copy with `copy()`

The copy() method is a member of the list class in Python. It’s used to create a new list that is a shallow copy of the original list. Here’s the basic syntax of the copy() method:

new_list = original_list.copy()

Let’s break down this syntax:

original_list is the list you want to copy.
.copy() is the method that creates a new list that’s a copy of original_list.
new_list is the new list that is created and contains the same elements as original_list.

It’s important to note that the elements within the new list are references to the same objects as in the original list. This means that changes made to the elements within the new list can affect the original list, and vice versa.

Shallow Copy vs. Deep Copy

Before diving into examples, let’s clarify the difference between a shallow copy and a deep copy.

A shallow copy creates a new list that holds references to the same objects as the original list. If the objects themselves are mutable (e.g., lists, dictionaries), changes made to these objects in the copied list will also be reflected in the original list.

On the other hand, a deep copy creates a new list and recursively creates copies of all the objects within the original list, ensuring that changes in the copied list do not affect the original list and vice versa. The copy module in Python provides the deepcopy() function to achieve this behavior.

In this tutorial, we’ll focus on creating shallow copies using the copy() method.

Examples of Using `copy()`

Example 1: Modifying Elements in the Shallow Copy

# Creating the original list
original_list = [1, 2, 3, [4, 5]]

# Creating a shallow copy using the copy() method
copied_list = original_list.copy()

# Modifying an element within the copied list
copied_list[3][0] = 6

# Checking the contents of both lists
print("Original List:", original_list)  # Output: [1, 2, 3, [6, 5]]
print("Copied List:", copied_list)      # Output: [1, 2, 3, [6, 5]]

In this example, we create a shallow copy of original_list using the copy() method. We then modify the first element of the nested list within the copied_list. As you can see, the change is reflected in both the original list and the copied list, since they share the same nested list object.

Example 2: Modifying the Copied List

# Creating the original list
original_list = [1, 2, 3]

# Creating a shallow copy using the copy() method
copied_list = original_list.copy()

# Modifying an element within the copied list
copied_list[0] = 4

# Checking the contents of both lists
print("Original List:", original_list)  # Output: [1, 2, 3]
print("Copied List:", copied_list)      # Output: [4, 2, 3]

In this example, we modify an element within the copied list. As expected, the change only affects the copied list and not the original list.

When to Use the copy() Method

The copy() method is useful when you want to create a new list that shares the same elements with an existing list. However, you should be aware of the implications of using shallow copies, especially when dealing with nested objects or mutable elements within the list.

Use the copy() method when:

You need a new list with the same elements as an existing list, and you want the changes made to one list to be reflected in the other.
You’re working with a simple list that doesn’t contain nested mutable objects.

Conclusion

In this tutorial, we’ve explored the copy() method for creating shallow copies of Python lists. We’ve learned that a shallow copy creates a new list with references to the same objects as the original list. This can lead to unexpected behavior when modifying nested mutable objects.

Understanding the difference between shallow and deep copies is essential for managing your data effectively and avoiding unintended side effects. The copy() method is a powerful tool when used appropriately, helping you manipulate and work with lists in Python.

Introduction to the copy() Method

Creating a Shallow Copy with copy()