Sentry Answers>Python>

Append vs. extend in Python

Append vs. extend in Python

David Y.

—

The ProblemJump To Solution

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.

  • Sentry BlogPython Performance Testing: A Comprehensive Guide
  • Sentry BlogLogging in Python: A Developer’s Guide
  • Syntax.fm logo
    Listen to the Syntax Podcast

    Tasty Treats for Web Developers brought to you by Sentry. Web development tips and tricks hosted by Wes Bos and Scott Tolinski

    Listen to Syntax

Loved by over 4 million developers and more than 90,000 organizations worldwide, Sentry provides code-level observability to many of the world’s best-known companies like Disney, Peloton, Cloudflare, Eventbrite, Slack, Supercell, and Rockstar Games. Each month we process billions of exceptions from the most popular products on the internet.

learn more about sentryjoin our discord
© 2024 • Sentry is a registered Trademark
of Functional Software, Inc.