How to Clone List in Python

Last updated on January 29th, 2025 at 05:11 am

Often software developers need to be able to create copies of lists in Python. This is commonly required if you do not want to modify existing lists but want to perform certain tasks on its copy. Typically, developers loop through the original list and append each of its items to an empty list, one by one. This is tedious and slow. There are several simple ways to copy list in Python. In this article, we will learn how to clone list in Python.

How to Clone List in Python

Here are the different ways to clone list in Python.

1. Using built-in copy() function

Since Python 3.3, every Python list comes with a built-in function copy() that can be used to clone a list in Python. Here is its syntax.

copy_list = original_list.copy()

Here is an example to copy list using this function.

a = [1, 2, 3, 4, 5]
b = a.copy()
print(b) # output is [1, 2, 3, 4, 5]

It provides a clean copy so that even if the original list is modified it does not affect the copied list.

a[1]=6
print(a) # output is [1, 6, 3, 4, 5]
print(b) # output is [1, 2, 3, 4, 5]

2. Using slicing method

Python provides slicing operation to extract a specific sub-list from a given list. Here is its syntax.

extract = original_list_or_string[start:end]

In the above command, slicing operator can be used on a list or a string. It accepts two optional parameters – start and end index to extract the sublist or substring.

It can also be used to create a copy of a list.

a = [1, 2, 3, 4, 5]
b = a[:]
print(b) # output is [1, 2, 3, 4, 5]

Here too it provides a clean copy that is not connected to original list.

a[1]=6
print(a) # output is [1, 6, 3, 4, 5]
print(b) # output is [1, 2, 3, 4, 5]

In fact, cloning list using slicing operator is the fastest way to copy a list. Although it has a weird syntax and is not the original purpose of slicing, it works great for all kinds of lists.

3. Using list() constructor

Python also provides a list constructor that accepts an iterable such as list, string, dict, etc. and returns a list. Here is its syntax.

list(iterable)

Here is an example to copy list using list() constructor.

a = [1, 2, 3, 4, 5]
b = list(a)
print(b) # output is [1, 2, 3, 4, 5]

This method also provides a completely new list, instead of providing a new reference to original list. Modifying original list will not affect the new list.

a[1]=6
print(a) # output is [1, 6, 3, 4, 5]
print(b) # output is [1, 2, 3, 4, 5]

Please note, list constructor only works with iterables. You cannot pass item values to it and get a list in return.

4. Using copy.copy() function

Python also provides a copy module that offers several functions to copy data. Among them copy() is a simple function that allows you to easily get a shallow copy of a Python object. Here is its syntax.

import copy
a = [1, 2, 3, 4, 5]
b = copy.copy(a)
print(b) # output is [1, 2, 3, 4, 5]

But in case of lists, it provides a clean copy that has no link to original list.

a[1]=6
print(a) # output is [1, 6, 3, 4, 5]
print(b) # output is [1, 2, 3, 4, 5]

5. Using list comprehension

You can also use list comprehension for this purpose. List comprehensions are reasonably fast and work well with small or big lists.

a = [1, 2, 3, 4, 5]
b = [i for i in a]
print(b) # output is [1, 2, 3, 4, 5]

This method also provides a separate copy of list.

a[1]=6
print(a) # output is [1, 6, 3, 4, 5]
print(b) # output is [1, 2, 3, 4, 5]

One advantage of this method is that you can customize your list comprehension to selectively copy one or more list items, instead of copying all items.

6. Using deepcopy() function

The copy module also provides deepcopy() function that allows you to do a deep copy of objects. You can also use it to clone lists. Here is its syntax.

copy_list = copy.deepcopy(original_list)

Here is an example to clone list using deepcopy.

import copy
a = [1, 2, 3, 4, 5]
b = copy.deepcopy(a)
print(b) # output is [1, 2, 3, 4, 5]

Since the cloned list is a deep copy, it does not change if the original list is modified.

a[1]=6
print(a) # output is [1, 2, 3, 4, 5]
print(b) // output is [1, 6, 3, 4, 5]

Please note, since deep copying requires more operations than shallow copying, it is slower than other solutions mentioned above.

If your list items are simple data types such as numbers and strings, then copy() and deepcopy() work same way. If your list contains complex objects such as dict or list of lists, then deepcopy() will be slower.

Conclusion

In this article, we have learnt several different ways to clone list in Python. Please note, in older Python versions, many of these solutions may have produced shallow copy where modifying the original list also modified the copy. But in recent Python versions, they all produce deep copies, where the cloned list has no link to the original list. Among them, using slicing method is the fastest and using copy.deepcopy() function is the slowest. Nevertheless, you can use any of the above solutions.

Also read:

How to Delete File or Folder in Python
How to Copy Files in Python
How to Iterate Over Rows in Pandas