Answers by Sentry

Append vs. extend in Python

David Y.

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.

Get Started With Sentry

Get actionable, code-level insights to resolve Python performance bottlenecks and errors.

  1. Create a free Sentry account

  2. Create a Python project and note your DSN

  3. Grab the Sentry Python SDK

pip install --upgrade sentry-sdk
  1. Configure your DSN
import sentry_sdk

sentry_sdk.init(
    "https://<key>@sentry.io/<project>",

    # Set traces_sample_rate to 1.0 to capture 100%
    # of transactions for performance monitoring.
    # We recommend adjusting this value in production.
    traces_sample_rate=1.0,
)

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
Share on Twitter
Bookmark this page
Ask a questionJoin the discussion

Related Answers

A better experience for your users. An easier life for your developers.

Try Sentry For FreeRequest a Demo
    TwitterGitHubDribbbleLinkedinDiscord
© 2024 • Sentry is a registered Trademark
of Functional Software, Inc.