Python List Extend Explained

In this tutorial, you’ll gain insights into the Python list Extend method, complete with illustrative examples showcasing its application on various sequences.

Important to note: The syntax provided in the following section pertains to Python 3, but it can be adapted to suit any other version of Python you may be using.

To Learn about Lists – Read Python List

Understand the List Extend Method

As we learned from previous tutorials, we can append elements to a list using the append method.

The append works fine when you want to add a single element or a list. But when you wish to append an individual letter from a word or an array of single-digit numbers, then it becomes impossible to achieve. Hence, the extend method comes into the scene to address this limitation.

Extend() Method Syntax

This method updates the list by adding elements to the end. They can be a word or a number, etc. When you call this method, it traverses through the arguments and pushes them into the list one by one to the tail end.

Hence, the number of elements appended is the same as the number of arguments passed. It takes only one parameter and does not have a return value.

Its syntax is as follows:

List_name.extend(element)

After the Python extend method gets called, you will get the updated list object.

Extend Method Time Complexity

It has a time complexity that is proportional to the length of the list that we want to add.

Extend() Function Flowchart

When we pass the element to the extend method as an argument, it gets iterated, and the value from each iteration gets appended to the list.

The flowchart below attempts to explain it in a diagram:

List Extend Method Examples

While you use this method, consider the following points in mind.

a. When you add a “list” or “set” to a list, each element in the list gets iterated and appended to the tail end.

b. When you add a “string” to a list, the letters of the string get iterated and appended to the tail.

List to a List

targetList = ["Python", "CSharp", "Java", "GoLang", "Angular"]
listToExtend = ["C", "C++"]
targetList.extend(listToExtend)
print(targetList)
# ['Python', 'CSharp', 'Java', 'GoLang', 'Angular', 'C', 'C++']

Set to a List

targetList = ['Spain', 'France', 'Italy', 'New Zealand']
listToExtend = {'Germany', 'Switzerland'}
print(type(listToExtend))
# <class 'set'>
targetList.extend(listToExtend)
print(targetList)
# ['Spain', 'France', 'Italy', 'New Zealand', 'Germany', 'Switzerland']

String to a List

stringsList = ['P', 'Q', 'R']
stringsList.extend('jklmn')
print(stringsList)
# ['P', 'Q', 'R', 'j', 'k', 'l', 'm', 'n']

Also Read: How to use Python List Insert() Method

Key Facts

Here’s a summary of important facts about the List Extend method in Python presented in a table format for easy understanding:

4 Exercises on Python List Extend Method

Here are some unique Python exercises where the list.extend() method can be useful:

Combine Two Lists

Problem#1: Given two lists, list1, and list2, combine them into a single list.

Solution:

list1 = [1, 2, 3]
list2 = [4, 5]

list1.extend(list2)

# Result: list1 = [1, 2, 3, 4, 5]

Add User Inputs to a List

Problem#2: Create an empty list, user_list, and use a loop to add three user inputs (e.g., names) to the list.

Solution:

user_list = []

for _ in range(3):
    name = input("Enter a name: ")
    user_list.extend([name])

print(user_list)
# Example input: "Alice," "Bob," "Charlie"
# Result: user_list = ["Alice", "Bob", "Charlie"]

Merge and Sort Numbers

Problem#3: Create two lists, even_numbers and odd_numbers, containing numbers. Merge them into a single list and then sort the merged list.

Solution:

even_numbers = [2, 4, 6]
odd_numbers = [1, 3, 5]

merged_numbers = []
merged_numbers.extend(even_numbers)
merged_numbers.extend(odd_numbers)
merged_numbers.sort()

print(merged_numbers)

# Result: merged_numbers = [1, 2, 3, 4, 5, 6]

Flatten a List

Problem#4: Given a list of lists, nested_list, use list.extend() to flatten it into a single list.

Solution:

nested_list = [[1, 2], [3, 4], [5, 6]]
flat_list = []

for sublist in nested_list:
    flat_list.extend(sublist)

print(flat_list)
# Result: flat_list = [1, 2, 3, 4, 5, 6]

Best,

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