Python Lists

So far, every variable you have created held one value: one name, one price, one score. But real programs deal with many values at once: all the songs in a playlist, every item in a cart, the top ten players in a game. Python lists let you store all of those values together under a single name. In this lesson, you'll learn how to create a list, read items by their position, slice a list, change items, and loop through every item.

What is a list in Python?

A list is an ordered collection of values stored in one variable. You write a list with square brackets [] and separate the items with commas. Each value inside a list is called an item (or an element).

groceries = ["milk", "eggs", "bread", "apples"]
print(groceries)
print(type(groceries))

['milk', 'eggs', 'bread', 'apples']
<class 'list'>

Notice that Python prints the whole list, brackets and all. The type of a list is list.

How do you create a list?

There are a few common ways to make a list:

scores = [88, 92, 79]          # a list of numbers
empty_cart = []                 # an empty list, ready to fill later
letters = list("cat")           # list() turns a string into a list of characters
numbers = list(range(1, 6))     # list() also works with range()

print(scores)
print(empty_cart)
print(letters)
print(numbers)

[88, 92, 79]
[]
['c', 'a', 't']
[1, 2, 3, 4, 5]

The built-in list() function builds a list from anything you can loop over, such as a string or a range.

Lists can mix types (and allow duplicates)

A single list can hold strings, numbers, booleans, even other lists. Items can also repeat.

profile = ["Maya", 29, True, 1.68]
rolls = [6, 2, 6, 6, 1]

print(profile)
print(rolls)

['Maya', 29, True, 1.68]
[6, 2, 6, 6, 1]

How does list indexing work?

Every item in a list has a position number called an index. Indexing starts at 0, not 1, exactly like the characters in a string. You read an item by putting its index in square brackets after the list name.

playlist = ["Intro", "Sunrise", "Echoes", "Drift", "Outro"]

print(playlist[0])   # first item
print(playlist[2])   # third item
print(playlist[4])   # fifth (last) item

Intro
Echoes
Outro

If you ask for an index that doesn't exist, Python stops with an error:

playlist = ["Intro", "Sunrise", "Echoes", "Drift", "Outro"]
print(playlist[5])

IndexError: list index out of range

Negative indexes: counting from the end

A negative index counts backward from the end of the list. -1 is the last item, -2 the second to last, and so on. This is handy when you want "the latest" or "the last" item without knowing how long the list is.

temperatures = [14, 16, 15, 19, 21]

print(temperatures[-1])   # most recent reading
print(temperatures[-2])   # the one before it

21
19

How do you slice a list?

Slicing takes a section of a list and gives you a new list. The syntax is list[start:stop], and it works the same way as string slicing: the start index is included, the stop index is not.

playlist = ["Intro", "Sunrise", "Echoes", "Drift", "Outro"]

print(playlist[1:4])    # items at index 1, 2 and 3
print(playlist[:2])     # from the start up to (not including) index 2
print(playlist[3:])     # from index 3 to the end
print(playlist[-2:])    # the last two items
print(playlist[::2])    # every second item
print(playlist[::-1])   # the whole list, reversed

['Sunrise', 'Echoes', 'Drift']
['Intro', 'Sunrise']
['Drift', 'Outro']
['Drift', 'Outro']
['Intro', 'Echoes', 'Outro']
['Outro', 'Drift', 'Echoes', 'Sunrise', 'Intro']

Unlike indexing, slicing never raises an error for out-of-range numbers. playlist[2:100] simply gives you everything from index 2 to the end.

Lists are mutable: changing items

Mutable means "can be changed after it's created". Strings are immutable (you can't change one letter in place), but lists are mutable. You can replace an item by assigning to its index:

groceries = ["milk", "eggs", "bread", "apples"]

groceries[2] = "bagels"      # swap bread for bagels
groceries[-1] = "bananas"    # change the last item

print(groceries)

['milk', 'eggs', 'bagels', 'bananas']

You can even replace a whole slice at once. The new section doesn't have to be the same length:

week = ["Mon", "Tue", "Wed", "Thu", "Fri"]
week[1:3] = ["Holiday"]
print(week)

['Mon', 'Holiday', 'Thu', 'Fri']

How do you count items and check membership?

Counting items with len()

The built-in len() function tells you how many items a list has. The last valid index is always len(list) - 1.

cart = ["T-shirt", "Sneakers", "Cap"]
print(len(cart))
print(len([]))

3
0

Checking if an item is in a list

Use the in operator (you met it in membership operators) to check whether a value is in a list. It returns True or False.

allergies = ["peanuts", "shellfish"]

print("peanuts" in allergies)
print("gluten" in allergies)
print("gluten" not in allergies)

True
False
True

How do you loop through a list?

A for loop visits every item in order, one at a time. This is the most common thing you'll do with a list.

guests = ["Liam", "Sofia", "Kenji"]

for guest in guests:
    print(f"Welcome, {guest}!")

Welcome, Liam!
Welcome, Sofia!
Welcome, Kenji!

If you also need the position, loop over range(len(...)), or use the cleaner built-in enumerate(), which gives you the index and the item together:

leaderboard = ["Ava", "Noah", "Zara"]

for position, player in enumerate(leaderboard, start=1):
    print(position, player)

1 Ava
2 Noah
3 Zara

Nested lists: a list inside a list

An item in a list can itself be a list. This is called a nested list, and it's a simple way to store rows of data, like a table or a game board. Use one index to pick the inner list, then a second index to pick an item inside it.

board = [
    ["X", "O", "X"],
    ["O", "X", "O"],
    ["O", "X", "X"],
]

print(board[0])       # the first row
print(board[1][2])    # row 1, column 2
board[2][0] = "X"     # change one square
print(board[2])

['X', 'O', 'X']
O
['X', 'X', 'X']

Real-world example: a game leaderboard

Here's a small program that keeps the top scores of a game, then prints the winner, the runner-up and the average score.

players = ["Ava", "Noah", "Zara", "Leo"]
scores = [1250, 980, 1430, 1100]

best = max(scores)
winner = players[scores.index(best)]

print(f"Players: {len(players)}")
print(f"Winner: {winner} with {best} points")
print(f"Average score: {sum(scores) / len(scores)}")

for i in range(len(players)):
    print(f"{players[i]:<6}{scores[i]:>6}")

Players: 4
Winner: Zara with 1430 points
Average score: 1190.0
Ava     1250
Noah     980
Zara    1430
Leo     1100

The built-in functions max(), min() and sum() work on any list of numbers, and scores.index(best) finds the position of the best score (you'll see index() again in the next lesson).

Common mistakes with lists

1. Forgetting that indexes start at 0

colors = ["red", "green", "blue"]
print(colors[3])

IndexError: list index out of range

A list with 3 items has indexes 0, 1 and 2. Use colors[-1] or colors[len(colors) - 1] for the last item.

2. Expecting the stop index to be included in a slice

days = ["Mon", "Tue", "Wed", "Thu"]
print(days[1:3])

['Tue', 'Wed']

The slice stops before index 3, so "Thu" is not included.

3. Thinking two names mean two lists

original = [1, 2, 3]
backup = original      # NOT a copy: both names point to the same list
backup[0] = 99
print(original)

[99, 2, 3]

Assignment never copies a list; it just adds another label to it (just like the variable labels you saw earlier). To make a real copy, use original[:] or original.copy().

Practice exercise

cities = ["Tokyo", "Paris", "Cairo", "Sydney", "Toronto"]

print(cities[0], cities[-1])
cities[2] = "Lisbon"
print(cities[1:4])

for number, city in enumerate(cities, start=1):
    print(f"{number}. {city}")

Tokyo Toronto
['Paris', 'Lisbon', 'Sydney']
1. Tokyo
2. Paris
3. Lisbon
4. Sydney
5. Toronto

What's next?

You can now create lists, read and change items, slice them and loop through them. Next, learn the built-in Python list methods for adding, removing, sorting and searching items.