How to Extend a List with Another List in Python

In Python, lists are fundamental data structures. Often, you’ll need to combine two lists into a single list.
This article explores several ways to extend a list with another list in Python, including the extend() method, the + operator, and list comprehensions.

We’ll provide detailed explanations, real-world examples, and practical use cases to help you master these techniques. Learn how to effectively use Python to manipulate and combine lists using the extend() method and other approaches.

Let’s start with a basic example of how we can extend lists. Say you have these lists:

list1 = [1, 2, 3]
list2 = [4, 5, 6]

print(f"List 1: {list1}")
print(f"List 2: {list2}")

List 1: [1, 2, 3]
List 2: [4, 5, 6]

Method 1: Using the extend() Method

The extend() method is the most straightforward way to add elements from one list to the end of another. It modifies the original list directly.

list1 = [1, 2, 3]
list2 = [4, 5, 6]

list1.extend(list2)

print(f"Extended List 1: {list1}")
print(f"List 2: {list2}") # List 2 remains unchanged

Extended List 1: [1, 2, 3, 4, 5, 6]
List 2: [4, 5, 6]

In this example, list1.extend(list2) adds all elements from list2 to the end of list1. Importantly, extend() modifies list1 in place, meaning it directly alters the original list. list2 is unchanged.

Method 2: Using the + Operator

The + operator creates a new list containing all elements from both lists. It does not modify the original lists.

list1 = [1, 2, 3]
list2 = [4, 5, 6]

list3 = list1 + list2

print(f"List 1: {list1}")
print(f"List 2: {list2}")
print(f"Concatenated List: {list3}")

List 1: [1, 2, 3]
List 2: [4, 5, 6]
Concatenated List: [1, 2, 3, 4, 5, 6]

Here, list1 + list2 creates a new list, list3, containing all elements from list1 followed by all elements from list2. The original lists, list1 and list2, remain unchanged. This method is useful when you want to create a new list without altering the original ones.

Method 3: Using List Comprehension (Less Common for Simple Extension)

While less common for directly extending a list, list comprehensions can be used to achieve similar results, especially when applying transformations during the extension process. This method is generally used when you want to conditionally add elements or modify them while combining the lists.

list1 = [1, 2, 3]
list2 = [4, 5, 6]

list3 = list1 + [x for x in list2] # Equivalent to list1 + list2 in this case

print(f"List 1: {list1}")
print(f"List 2: {list2}")
print(f"Concatenated List: {list3}")

List 1: [1, 2, 3]
List 2: [4, 5, 6]
Concatenated List: [1, 2, 3, 4, 5, 6]

In this example, the list comprehension [x for x in list2] simply iterates through list2 and includes each element x in the new list. Combined with the + operator, this achieves the same result as direct concatenation. However, this approach becomes more powerful when you add conditions or transformations within the list comprehension.

Method 4: Extending with Different Data Types (extend() handles iterables)

The extend() method is versatile because it can extend a list with any iterable, such as tuples or strings.

list1 = [1, 2, 3]
tuple1 = (4, 5, 6)
string1 = "abc"

list1.extend(tuple1)
print(f"Extended with Tuple: {list1}")

list1.extend(string1)
print(f"Extended with String: {list1}")

Extended with Tuple: [1, 2, 3, 4, 5, 6]
Extended with String: [1, 2, 3, 4, 5, 6, 'a', 'b', 'c']

Here, list1 is extended first with a tuple and then with a string. When extending with a string, each character is treated as a separate element and added to the list.

Method 5: Extending a List in Place using += Operator

The += operator provides a shorthand way to extend a list in place, similar to the extend() method.

list1 = [1, 2, 3]
list2 = [4, 5, 6]

list1 += list2

print(f"Extended List 1: {list1}")

Extended List 1: [1, 2, 3, 4, 5, 6]

The list1 += list2 statement is equivalent to list1 = list1 + list2. However, unlike the + operator, += modifies list1 directly, behaving like the extend() method.

Choosing the Right Method

• Use extend() or += when you want to modify the original list directly. They are generally more efficient for extending lists in place.
• Use the + operator when you want to create a new list without changing the original lists. This is useful for preserving the original data.
• Use list comprehensions when you need to apply transformations or filters while combining lists. They provide more flexibility for complex scenarios.