Python lists

What is a list?

A list stores many values in one place. In Python, we write a list with square brackets [ ].

Imagine you want to save the names of four students. You could make four variables. That is a lot of typing! A list is much shorter.

student1 = "Ana"
student2 = "Budi"
student3 = "Citra"
student4 = "Dewi"

students = ["Ana", "Budi", "Citra", "Dewi"]

Four things to know about lists

• Ordered. Items stay in the order you put them. The first item stays first.
• Changeable. You can change, add and remove items after you make the list. Python calls this mutable.
• Any type. A list can hold text, numbers, True/False, and even other lists. You can mix them.
• Duplicates are OK. The same value can appear many times: [1, 1, 2] is fine.

Lists in real life

You already use lists every day. Programmers use Python lists to build things like these:

• A music playlist. Songs in order. You can add, remove and skip songs.
• The contacts on your phone. A long list of names.
• A game backpack. Sword, shield, potion. Pick up items, use items.
• A to-do list. Add a task. Tick it off. Remove it.
• Class test scores. Find the best score and the average.

1Create a list

To make a list, put your items inside square brackets. Put a comma between the items.

Rules for writing items

• Text needs quotes: "apple" or 'apple'.
• Numbers do not need quotes: 90, 3.5.
• True and False start with a capital letter and have no quotes.
• Choose a clear name, like fruits or scores. No spaces. Use _ if you need two words, like my_list.

Kinds of lists

A list can hold many kinds of values. Pick a tab, press Run, then try to change the code!

fruits = ["apple", "banana", "cherry"]
print(fruits)

scores = [90, 85, 77]
prices = [1.5, 2.25, 3.0]
print(scores)
print(prices)

answers = [True, False, True]
print(answers)

student = ["Rina", 13, 1.55, True]
print(student)
print(type(student))

todo = []
print(todo)
print(len(todo))

class_a = [["Ana", 90], ["Budi", 85]]
print(class_a)

More ways to make a list

letters = list("hey")          # split text into letters
print(letters)

numbers = list(range(1, 6))    # 1 to 5 (stops BEFORE 6)
print(numbers)

zeros = [0] * 5                # repeat an item 5 times
print(zeros)

both = ["a", "b"] + ["c", "d"] # join two lists
print(both)

2Access items

To open a locker, write the list name and then the locker number inside [ ]. This number is called the index.

fruits = ["apple", "banana", "cherry", "mango", "grape"]
print(fruits[0])   # the first item
print(fruits[2])   # the third item
print(fruits[4])   # the fifth (last) item

Why 0? Think of the index as "how many steps from the start". The first item is 0 steps away.

Negative index: count from the end

A negative number counts from the right side. -1 is the last item, -2 is the second from last. The small numbers under each locker are the negative indexes.

fruits = ["apple", "banana", "cherry", "mango", "grape"]
print(fruits[-1])
print(fruits[-2])

How many items? Use len()

len() tells you how many items are in a list. The last index is always len() - 1.

fruits = ["apple", "banana", "cherry", "mango", "grape"]
print(len(fruits))
last_index = len(fruits) - 1
print(last_index)
print(fruits[last_index])

Slicing: get a few items

A slice takes a piece of the list. Write [start:stop]. Python starts at start and stops just before stop. The stop locker is not included!

fruits = ["apple", "banana", "cherry", "mango", "grape"]
print(fruits[1:3])   # index 1 and 2 (stops BEFORE 3)
print(fruits[:2])    # from the start
print(fruits[2:])    # to the end
print(fruits[-2:])   # the last two

A list inside a list

Use two indexes to go inside. The first opens the big locker. The second opens a small locker inside it.

classroom = [["Ana", "Budi"], ["Citra", "Dewi"]]
print(classroom[0])
print(classroom[1][0])

What if the locker does not exist?

If you use an index that is too big, Python shows an IndexError. Press Run to see it. Then change pets[3] to pets[2].

pets = ["cat", "dog", "fish"]
print(pets[0])
print(pets[3])

for pet in pets:
    print(pet)

3Update data

Lists are mutable. That means you can change them after you make them. You can put new things in the lockers, add new lockers, or remove lockers.

Change an item

Use the index and =. The old item is replaced by the new one.

colors = ["red", "green", "blue"]
colors[1] = "yellow"
print(colors)

Add items

• append(x) adds x at the end.
• insert(i, x) puts x at index i. The other items move to the right.
• extend(list) adds many items at the end.

snacks = ["chips", "cookies"]
snacks.append("candy")
print(snacks)

snacks.insert(1, "popcorn")
print(snacks)

snacks.extend(["nuts", "raisins"])
print(snacks)

Remove items

• remove(x) removes the first item that has the value x.
• pop() removes the last item and gives it back. pop(i) removes the item at index i.
• del list[i] deletes the item at index i.

pets = ["cat", "dog", "fish", "bird", "hamster"]
pets.remove("fish")      # remove by VALUE
print(pets)

last = pets.pop()        # remove the LAST item and keep it
print(last)
print(pets)

del pets[0]              # remove by INDEX
print(pets)

Put the list in order

numbers = [30, 10, 20]
numbers.sort()                 # small to big
print(numbers)

numbers.reverse()              # flip the order
print(numbers)

names = ["Zara", "Ali", "Mia"]
names.sort()                   # A to Z
print(names)

names.sort(reverse=True)       # Z to A
print(names)

Which tool should I use?

Real-life cases

Let us use what you learned to solve real problems. Press Run, then change the code to see what happens.

playlist = ["Sunrise", "Ocean Eyes", "Skyline"]
print("Songs:", playlist)

playlist.append("Dream Big")        # add a song at the end
playlist.insert(0, "Intro Beat")    # put a song first
playlist[2] = "Ocean Eyes (Remix)"  # change a song
playlist.remove("Skyline")          # delete a song

print("Now playing:", playlist[0])
print("Last song:", playlist[-1])
print("Total songs:", len(playlist))
print(playlist)

order = ["fried rice", "iced tea"]
order.append("meatball soup")
order.extend(["tempeh", "orange juice"])
print("My order:", order)
print("Items:", len(order))

# I changed my mind!
order[1] = "mineral water"
order.remove("tempeh")
print("Final order:", order)

scores = [78, 85, 92, 66, 88]
print("Scores:", scores)
print("Best score:", max(scores))
print("Lowest score:", min(scores))
print("Average:", sum(scores) / len(scores))

# Remedial test! Update the lowest score
scores[3] = 80
print("New average:", sum(scores) / len(scores))

scores.sort()
print("From low to high:", scores)

bag = ["sword", "shield", "potion"]
print("Slot 0 has:", bag[0])

bag.append("map")        # pick up a map
bag.insert(1, "bow")     # put a bow in slot 1
print("Bag:", bag)

used = bag.pop()         # use the last item
print("Used:", used)

bag[0] = "super sword"   # upgrade!
print("Bag now:", bag)
print("Is there a potion?", "potion" in bag)

Oops! Common mistakes

Everyone makes these mistakes. Open each one, run the code, and read the message. Error messages are your friends!

fruits = ["apple", "banana", "cherry"]
print(fruits[3])

names = [Ana, Budi]
print(names)

fruits = ["apple", "banana"]
print(fruits(0))

fruits = ["apple", "banana"]
fruits.remove("mango")

x = [1, 2, 3]
x.append([4, 5])
print(x)
print(len(x))

y = [1, 2, 3]
y.extend([4, 5])
print(y)
print(len(y))

scores = [30, 10, 20]
scores = scores.sort()
print(scores)

name = "Ana"
name[0] = "E"

a = ["x", "y"]
b = a              # NOT a copy!
b.append("z")
print(a)

c = a.copy()       # this IS a copy
c.append("w")
print(a)
print(c)

Playground

Write your own Python code here. Every list you make will appear as lockers below.

Missions

Six coding missions. Write the code, then press Run & check. Stuck? Use the hint!

Quick quiz

Eleven questions. You get feedback after each answer.

Cheat sheet

Here fruits = ["apple", "banana", "cherry"].