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Append vs. extend in Python

Append vs. extend in Python

David Y.

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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:

Click to Copy
# 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.

Click to Copy
# 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:

Click to Copy
# 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.

Click to Copy
# 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.

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