List basics

Estimated Time: 1 hour

What is a list?

In your life, you’ve come across all kinds of lists - grocery lists, todo lists, the attendance roll at school, the roster of your football team. Lists in Python are a way to represent many pieces of data, like you do on those lists. Instead of just one number, string, or boolean, a list can hold many pieces of data at once.

When working with lists in your life, there are a few core actions you perform:

• Create a list
• Add something to the list
• Remove something from the list
• Change an item on the list
• Take some action for all of the items on the list

In this lesson, we’ll cover the syntax for creating lists and updating lists, and performing these core actions.

List Syntax

A list is a sequence of values. It’s written like this:

[1,2,3]

The list is surrounded by square brackets, with elements separated by commas. The values inside a list are called ‘elements’ or ‘items’. You can assign lists to variables, just like you can with numbers and strings:

my_list = [1,2,3]

Lists can be empty. An empty list looks like this:

empty_list = []

Lists can contain any type of element, even other lists.

Below are some examples of valid lists:

countries = ["Kenya", "Ghana", "Ethiopia", "Zimbabwe"]
primes = [2, 3, 5, 7, 11]
empty_list = []
foods = [["apples", "kiwis", "bananas"], ["chorizo", "steak", "chicken nuggets"], ["ice cream", "popcorn", "chocolate bars"]]
different_types = [False, 1, "string", []]

The list data type

Earlier, you learned the basic Python data types: String, Int, Float, Boolean. A List is another core data type in Python, but it’s different from the others you’ve learned so far.

my_list = [1,2,3]
type(my_list) # <class 'list'>

A list is a type of data structure.

Different data structures are useful for solving different kinds of problems in Python. The other most common built-in Python data structures are dictionaries, sets, and tuples. We’re going to focus on Lists, but as you grow in your Python skills, you’ll learn the uses of these other types.

List indices

Each element of the list has a position, called an index. We can get an element from a list using the name of the list and its index in brackets. For example, given the list below:

countries = ['Kenya', 'Ghana', 'Ethiopia', 'Zimbabwe']

We can access the first list item using countries[0], the second list item using countries[1], and so on.

countries[0] # "Kenya"
countries[1] # "Ghana"

Remember that lists are indexed starting at 0. So the first item has position 0, the second item has position 1, and so on. This start-from-zero numbering is common to almost all programming languages.

countries[0] # "Kenya"
countries[2] # "Ethiopia"

After you access an element from a list, you can use it anywhere you’d use another value or variable.

print(countries[0]) # Kenya
print(countries[1] + " is a nice place to visit") # Ghana is a nice place to visit

Practice: Access List Items

tallest_buildings = ["Burj Khalifa", "Merdeka 118", "Shanghai Tower", "Abraj Al-Bait Clock Tower", "Ping An International Finance Centre", "Lotte World Tower"]

print(tallest_buildings[0])
print(tallest_buildings[2])
print(tallest_buildings[1], "is taller than", tallest_buildings[5])

More about lists

IndexError

If you try to access a value past the end of the list, Python will raise an error:

print(countries) # ['Kenya', 'Ghana', 'Ethiopia', 'Zimbabwe']
countries[3] # "Zimbabwe"
countries[4] # IndexError: list index out of range

len

To keep from going out of bounds, you can get the length of the list with the function len:

len(countries) # 4

Last item

If there are 4 elements, then the last index is 3.

Since we start counting at 0, the last element is always at index one less than the length of the list.

countries = ['Kenya', 'Ghana', 'Ethiopia', 'Zimbabwe']
last_index = len(countries) - 1 # 3
countries[last_index] # "Zimbabwe"
countries[len(countries) - 1] # "Zimbabwe"

Negative indexes

Python has a shorthand for accessing elements from the end of the list:

countries[-1] # "Zimbabwe"
countries[-2] # "Ethiopia"

Negative indices start counting from the end of the list, so -1 is the last index, and -2 is the second-to-last, and so on.