Append vs. extend in Python

The Problem

What is the difference between Python’s list methods append() and extend()?

The Solution

The append() method is used to add an item to the end of a list, whereas the extend() method is used to merge a second list (or other iterable) onto the end of a list. Consider the following examples:

# Use of append
products = ["Apples", "Oranges", "Bananas"]
new_product = "Apricots"

products.append(new_product)
print(products)  # will output ['Apples', 'Oranges', 'Bananas', 'Apricots']

In the code above, a single item was appended to the list.

# Use of extend
products = ["Apples", "Oranges", "Bananas"]
new_products = ["Apricots", "Mangoes"]

products.extend(new_products)
print(products)  # will output ['Apples', 'Oranges', 'Bananas', 'Apricots', 'Mangoes']

In the code above, the list was extended with a second list.

Now let’s see what happens if we swap the methods:

# Incorrect use of extend
products = ["Apples", "Oranges", "Bananas"]
new_product = "Apricots"

products.extend(new_product)
print(products)  # will output ['Apples', 'Oranges', 'Bananas', 'A', 'p', 'r', 'i', 'c', 'o', 't', 's']

Here, our use of extend() has caused each element (i.e. character) of the new_product string to be added to our list individually.

# Incorrect use of append
products = ["Apples", "Oranges", "Bananas"]
new_products = ["Apricots", "Mangoes"]

products.append(new_products)
print(products)  # will output ['Apples', 'Oranges', 'Bananas', ['Apricots', 'Mangoes']]

Here, our use of append() has caused the new_products list to be treated as a single item.

In summary, we should use append() when we want to add a single item to the end of a list and extend() when we want to merge our list with another.