Python Copy List: What You Should Know

Python lists are powerful data structures used to store a collection of items. The copy list method is important when handling data since it helps preserve the original list while performing operations on the duplicated one. In this tutorial, I will explain the basic and advanced techniques of copying lists with practical examples.

Before exploring the different examples, I recommend taking DataCamp’s Introduction to Python course to learn the basics of Python, including the different Python data structures. The Python Cheat Sheet for Beginners will also be helpful for referencing different functions and methods.

"Python using a copy machine." Image by Dall-E.

Quick Answer: How to Copy a List in Python

The example below shows how to copy a list in Python using the copy() method. Notice how the original list remains unchanged even though changes are made to the duplicate list. The code will print [1, 2, 3].

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

# Create a shallow copy of the original list
copied_list = original_list.copy()

# Modify the first element of the copied list
copied_list[0] = 99

print(original_list)

[1, 2, 3]

More Techniques to Copy a List in Python

Before we look at the different techniques for copying a list in Python, let’s consider why we don’t use the assignment operator =. We don’t use = because doing so would not actually create a copy of our list. The assignment operator would only create a reference to our list. 

In the following example, original_list and reference_to_original_list refer to the same list after our assignment.

# Create a list called original_list
original_list = [1, 2, 3]

# Create a new reference copied_list that points to the same list as original_list
reference_to_original_list = original_list

Here, any modifications on the duplicated list will affect the original list and vice versa. These changes occur since the two lists are stored in the same memory. Now, let’s take a look at the different ways to actually copy our list. 

Using the copy() method

The copy() method is a built-in method used to create a shallow copy of the list. The copy() method is available from Python 3.3 onwards. This method copies only the top-level list structure. The syntax of the copy() method is shown below.

new_list = original_list.copy()

In the following example, the code will print [1, 2, 3, 4].

# Original list with nested structure
original_list = [1, 2, 3, 4]

# Create a shallow copy using the copy() method
shallow_copied_list = original_list.copy()

print(shallow_copied_list)

[1, 2, 3, 4]

Using the list() constructor

The list() constructor is used to create a new list from an existing list to include all the elements from the original list. This technique is important when creating a new list object where changes in the copied list will not affect the original list. The following code prints [1, 2, 3].

# Create a list called original_list
original_list = [1, 2, 3]

# Create a shallow copy of original_list using the list() function
copied_list = list(original_list)

# Modify the first element of copied_list
copied_list[0] = 99

print(original_list)

[1, 2, 3]

Using list slicing with [:]

List slicing [:] provides a concise method of creating a shallow copy of a list. The list slicing [:] method is efficient since it is implemented at a low level in Python. Below is the syntax of this method. 

new_list = original_list[:]

The following code prints [1, 2, 3].

# Create a list called original_list
original_list = [1, 2, 3]

# Create a shallow copy of original_list using list slicing
copied_list = original_list[:]

# Modify the first element of copied_list
copied_list[0] = 99

print(original_list)

[1, 2, 3]

In this context, we use the list slicing method to include everything in the list, but in other contexts, slicing can be used to create sublists by specifying start and end indices.