Adding a List to Another List in Python

Combining lists is a common operation in Python. Here’s how to add one list to another using methods like extend() and append().

Using extend() to Add a List

The extend() method adds each element from one list to another. It’s the most efficient way to merge lists in Python.

list_a = [1, 2, 3]
list_b = [4, 5, 6]
list_a.extend(list_b)
print(list_a)

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

In this example, extend() combines list_b into list_a, making a single list.

Using + Operator to Concatenate Lists

The + operator also joins lists by creating a new list without changing the original lists:

list_a = [1, 2, 3]
list_b = [4, 5, 6]
combined_list = list_a + list_b
print(combined_list)

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

Using + is straightforward but creates a new list, unlike extend() which modifies the original list.

Using append() for Nested Lists

To add a list as a single element, use append(). This creates a nested list structure:

list_a = [1, 2, 3]
list_b = [4, 5, 6]
list_a.append(list_b)
print(list_a)

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

Here, list_b becomes an element inside list_a. Use append() if you need a nested list structure.

For more on appending lists, check Python Spread List Append: Adding Multiple Items Efficiently.

Using List Comprehension

List comprehension allows adding elements conditionally:

list_a = [1, 2, 3]
list_b = [item for item in [4, 5, 6] if item > 4]
list_a.extend(list_b)
print(list_a)

[1, 2, 3, 5, 6]

Only elements greater than 4 are added to list_a. This approach provides flexibility in filtering items.

Conclusion

Python offers several ways to add lists together, each with unique benefits. Choose extend() for modifying lists directly, + for a new list, or append() for nesting.

For more list operations, see Python's official documentation.