Lists

Data Structures: Lists

A list stores a collection of items in a single variable. Lists are useful when the number of items may change, such as a playlist, a shopping basket, or a class roster.

Creating Lists

Create a list by placing comma-separated items inside square brackets [].

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

print(fruits)

['apple', 'banana', 'cherry']

print(type(fruits))

<class 'list'>

A list can contain any type of data. It can also mix types, although lists are often easier to work with when their items represent the same kind of information.

profile = ["Maya", 21, True]

print(profile)

['Maya', 21, True]

A list can contain other lists. This is called a nested list.

weekly_schedule = [
    ["Python", "Design"],
    ["Economics", "Writing"],
    ["Statistics", "Python"],
]

print(weekly_schedule)

[['Python', 'Design'], ['Economics', 'Writing'], ['Statistics', 'Python']]

Properties of Lists

Ordered

Items remain in a specific order. Python uses zero-based indexing, so the first item is at index 0.

fruits = ["apple", "banana", "cherry", "pineapple", "watermelon"]

print(fruits[0])

apple

Indexing lists works exactly like strings, which are also ordered sequences.

print(fruits[1])

banana

Negative indexes count from the end. The index -1 refers to the last item.

print(fruits[-1])

watermelon

Slicing

Slicing selects part of a list. The starting index is included and the ending index is excluded.

print(fruits[1:4])

['banana', 'cherry', 'pineapple']

print(fruits[:2])

['apple', 'banana']

print(fruits[3:])

['pineapple', 'watermelon']

Use more than one index to access an item inside a nested list.

weekly_schedule = [
    ["Python", "Design"],
    ["Economics", "Writing"],
    ["Statistics", "Python"],
]

print(weekly_schedule[1])

['Economics', 'Writing']

print(weekly_schedule[1][0])

Economics

Exercise: Predict the output

Without running the code, write the three lines that it prints.

shows = ["Severance", "The Bear", "Andor", "Arcane"]

print(shows[0])
print(shows[-2])
print(shows[1:3])

Solution

Severance
Andor
['The Bear', 'Andor']

Mutable

Lists are mutable, which means their contents can change after creation.

fruits = ["apple", "banana", "cherry"]
fruits[0] = "orange"

print(fruits)

['orange', 'banana', 'cherry']

Duplicate Items

Lists can contain the same value more than once.

votes = ["blue", "green", "blue", "red", "blue"]

print(votes)

['blue', 'green', 'blue', 'red', 'blue']

Operations with Lists

Lists are sequences, like strings. The + operator joins two lists.

morning_tasks = ["check email", "attend class"]
afternoon_tasks = ["study", "go to the gym"]

all_tasks = morning_tasks + afternoon_tasks
print(all_tasks)

['check email', 'attend class', 'study', 'go to the gym']

Both operands must be lists. A single item must be placed inside a list before it can be concatenated.

tasks = ["check email", "attend class"]
tasks = tasks + "study"

TypeError: can only concatenate list (not "str") to list

tasks = ["check email", "attend class"]
tasks = tasks + ["study"]

print(tasks)

['check email', 'attend class', 'study']

Multiplying a list repeats its items.

notification_pattern = ["work", "break"] * 3

print(notification_pattern)

['work', 'break', 'work', 'break', 'work', 'break']

The len() function returns the number of items in a list.

fruits = ["apple", "banana", "cherry", "pineapple", "watermelon"]

print(len(fruits))

For a nested list, len() counts only the items in the outer list.

weekly_schedule = [
    ["Python", "Design"],
    ["Economics", "Writing"],
    ["Statistics", "Python"],
]

print(len(weekly_schedule))
print(len(weekly_schedule[0]))

3
2

Exercise: Conference schedule

The list below stores two conference days. Write code that prints:

The number of days in the schedule.
The number of sessions on day 2.
The title of the first session on day 2.

schedule = [
    ["Opening", "Data Ethics", "Networking"],
    ["Python Lab", "Career Panel", "Closing"],
]

Solution

print(len(schedule))
print(len(schedule[1]))
print(schedule[1][0])

Built-in List Methods

Most list methods modify the original list in place. They do not usually need to be assigned back to the variable.

Adding Items

append() adds one item to the end of a list.

playlist = ["Intro", "Night Drive"]
playlist.append("Last Train")

print(playlist)

['Intro', 'Night Drive', 'Last Train']

extend() adds every item from another iterable.

playlist = ["Intro", "Night Drive"]
new_tracks = ["Last Train", "Home"]
playlist.extend(new_tracks)

print(playlist)

['Intro', 'Night Drive', 'Last Train', 'Home']

insert() adds an item at a specified index.

playlist.insert(1, "Interlude")

print(playlist)

['Intro', 'Interlude', 'Night Drive', 'Last Train', 'Home']

Warning

append(["A", "B"]) adds one nested list. extend(["A", "B"]) adds two separate items.

Exercise: Build a watchlist

Start with the list below and use list methods to make these changes:

Add "Arrival" to the end.
Insert "Spirited Away" at the beginning.
Add both "Moonlight" and "Dune" in one operation.

watchlist = ["Past Lives", "The Matrix"]

Solution

watchlist = ["Past Lives", "The Matrix"]

watchlist.append("Arrival")
watchlist.insert(0, "Spirited Away")
watchlist.extend(["Moonlight", "Dune"])

print(watchlist)

Removing Items

remove() removes the first occurrence of a value.

guests = ["Ava", "Leo", "Nora", "Leo"]
guests.remove("Leo")

print(guests)

['Ava', 'Nora', 'Leo']

pop() removes and returns an item. With no index, it removes the last item.

guests = ["Ava", "Leo", "Nora", "Sam"]
cancelled_guest = guests.pop(1)

print(cancelled_guest)
print(guests)

Leo
['Ava', 'Nora', 'Sam']

clear() removes all items.

guests.clear()

print(guests)

[]

Finding and Counting Items

index() returns the index of the first matching item. count() returns how many times a value appears.

responses = ["yes", "no", "yes", "maybe", "yes"]

print(responses.index("maybe"))
print(responses.count("yes"))

3
3

Sorting and Reversing

sort() sorts the original list. reverse() reverses its current order.

cities = ["Tokyo", "Lima", "Berlin", "Accra"]
cities.sort()

print(cities)

cities.reverse()
print(cities)

['Accra', 'Berlin', 'Lima', 'Tokyo']
['Tokyo', 'Lima', 'Berlin', 'Accra']

The key argument lets us choose what Python uses for sorting.

def last_character(word):
    return word[-1]


animals = ["Dog", "Cat", "Bird", "Horse"]
animals.sort(key=last_character)

print(animals)

['Bird', 'Horse', 'Dog', 'Cat']

Exercise: Sort by length

Sort the podcast titles from shortest to longest. Use len as the sorting key.

podcasts = ["Radiolab", "Today Explained", "Serial", "99% Invisible"]

Solution

podcasts = ["Radiolab", "Today Explained", "Serial", "99% Invisible"]
podcasts.sort(key=len)

print(podcasts)

Exercise: Fix the program

The programmer wants to add two destinations separately, but the result is not correct. Fix one line.

destinations = ["Lisbon", "Seoul"]
destinations.append(["Cairo", "Oslo"])

print(destinations)

Solution

Use extend() so that both destinations become individual items.

destinations = ["Lisbon", "Seoul"]
destinations.extend(["Cairo", "Oslo"])

print(destinations)

Challenge: Reading tracker

Complete a small reading tracker that:

Starts with three book titles.
Adds a fourth title.
Removes one completed book and stores its title in finished_book.
Sorts the remaining titles alphabetically.
Prints the finished book, the updated list, and the number of books still waiting.

Possible solution

reading_list = ["Beloved", "Dune", "The Hobbit"]

reading_list.append("Pachinko")
finished_book = reading_list.pop(1)
reading_list.sort()

print(f"Finished: {finished_book}")
print(f"Still to read: {reading_list}")
print(f"Books remaining: {len(reading_list)}")

Summary

Lists use square brackets: [].
Lists are ordered and mutable.
Lists allow duplicate items.
Indexes and slices retrieve items.
Methods such as append(), remove(), and sort() modify a list in place.

--- title: "Lists" format: html execute: error: true --- # Data Structures: Lists A **list** stores a collection of items in a single variable. Lists are useful when the number of items may change, such as a playlist, a shopping basket, or a class roster. ## Creating Lists Create a list by placing comma-separated items inside square brackets `[]`. ```{python} fruits = ["apple", "banana", "cherry"] print(fruits) ``` ```{python} print(type(fruits)) ``` A list can contain any type of data. It can also mix types, although lists are often easier to work with when their items represent the same kind of information. ```{python} profile = ["Maya", 21, True] print(profile) ``` A list can contain other lists. This is called a **nested list**. ```{python} weekly_schedule = [ ["Python", "Design"], ["Economics", "Writing"], ["Statistics", "Python"], ] print(weekly_schedule) ``` ## Properties of Lists ### Ordered Items remain in a specific order. Python uses **zero-based indexing**, so the first item is at index `0`. ```{python} fruits = ["apple", "banana", "cherry", "pineapple", "watermelon"] print(fruits[0]) ``` Indexing lists works exactly like strings, which are also ordered sequences. ```{python} print(fruits[1]) ``` Negative indexes count from the end. The index `-1` refers to the last item. ```{python} print(fruits[-1]) ``` ### Slicing Slicing selects part of a list. The starting index is included and the ending index is excluded. ```{python} print(fruits[1:4]) ``` ```{python} print(fruits[:2]) ``` ```{python} print(fruits[3:]) ``` Use more than one index to access an item inside a nested list. ```{python} weekly_schedule = [ ["Python", "Design"], ["Economics", "Writing"], ["Statistics", "Python"], ] print(weekly_schedule[1]) ``` ```{python} print(weekly_schedule[1][0]) ``` ::: {.callout-note} ## Exercise: Predict the output Without running the code, write the three lines that it prints. ```python shows = ["Severance", "The Bear", "Andor", "Arcane"] print(shows[0]) print(shows[-2]) print(shows[1:3]) ``` ::: ::: {.callout-tip collapse="true"} ## Solution ```text Severance Andor ['The Bear', 'Andor'] ``` ::: ### Mutable Lists are **mutable**, which means their contents can change after creation. ```{python} fruits = ["apple", "banana", "cherry"] fruits[0] = "orange" print(fruits) ``` ### Duplicate Items Lists can contain the same value more than once. ```{python} votes = ["blue", "green", "blue", "red", "blue"] print(votes) ``` ## Operations with Lists Lists are sequences, like strings. The `+` operator joins two lists. ```{python} morning_tasks = ["check email", "attend class"] afternoon_tasks = ["study", "go to the gym"] all_tasks = morning_tasks + afternoon_tasks print(all_tasks) ``` Both operands must be lists. A single item must be placed inside a list before it can be concatenated. ```{python} #| error: true tasks = ["check email", "attend class"] tasks = tasks + "study" ``` ```{python} tasks = ["check email", "attend class"] tasks = tasks + ["study"] print(tasks) ``` Multiplying a list repeats its items. ```{python} notification_pattern = ["work", "break"] * 3 print(notification_pattern) ``` The `len()` function returns the number of items in a list. ```{python} fruits = ["apple", "banana", "cherry", "pineapple", "watermelon"] print(len(fruits)) ``` For a nested list, `len()` counts only the items in the outer list. ```{python} weekly_schedule = [ ["Python", "Design"], ["Economics", "Writing"], ["Statistics", "Python"], ] print(len(weekly_schedule)) print(len(weekly_schedule[0])) ``` ::: {.callout-note} ## Exercise: Conference schedule The list below stores two conference days. Write code that prints: 1. The number of days in the schedule. 2. The number of sessions on day 2. 3. The title of the first session on day 2. ```python schedule = [ ["Opening", "Data Ethics", "Networking"], ["Python Lab", "Career Panel", "Closing"], ] ``` ::: ::: {.callout-tip collapse="true"} ## Solution ```python print(len(schedule)) print(len(schedule[1])) print(schedule[1][0]) ``` ::: ## Built-in List Methods Most list methods modify the original list **in place**. They do not usually need to be assigned back to the variable. ### Adding Items `append()` adds one item to the end of a list. ```{python} playlist = ["Intro", "Night Drive"] playlist.append("Last Train") print(playlist) ``` `extend()` adds every item from another iterable. ```{python} playlist = ["Intro", "Night Drive"] new_tracks = ["Last Train", "Home"] playlist.extend(new_tracks) print(playlist) ``` `insert()` adds an item at a specified index. ```{python} playlist.insert(1, "Interlude") print(playlist) ``` ::: {.callout-warning} `append(["A", "B"])` adds one nested list. `extend(["A", "B"])` adds two separate items. ::: ::: {.callout-note} ## Exercise: Build a watchlist Start with the list below and use list methods to make these changes: 1. Add `"Arrival"` to the end. 2. Insert `"Spirited Away"` at the beginning. 3. Add both `"Moonlight"` and `"Dune"` in one operation. ```python watchlist = ["Past Lives", "The Matrix"] ``` ::: ::: {.callout-tip collapse="true"} ## Solution ```python watchlist = ["Past Lives", "The Matrix"] watchlist.append("Arrival") watchlist.insert(0, "Spirited Away") watchlist.extend(["Moonlight", "Dune"]) print(watchlist) ``` ::: ### Removing Items `remove()` removes the first occurrence of a value. ```{python} guests = ["Ava", "Leo", "Nora", "Leo"] guests.remove("Leo") print(guests) ``` `pop()` removes and returns an item. With no index, it removes the last item. ```{python} guests = ["Ava", "Leo", "Nora", "Sam"] cancelled_guest = guests.pop(1) print(cancelled_guest) print(guests) ``` `clear()` removes all items. ```{python} guests.clear() print(guests) ``` ### Finding and Counting Items `index()` returns the index of the first matching item. `count()` returns how many times a value appears. ```{python} responses = ["yes", "no", "yes", "maybe", "yes"] print(responses.index("maybe")) print(responses.count("yes")) ``` ### Sorting and Reversing `sort()` sorts the original list. `reverse()` reverses its current order. ```{python} cities = ["Tokyo", "Lima", "Berlin", "Accra"] cities.sort() print(cities) cities.reverse() print(cities) ``` The `key` argument lets us choose what Python uses for sorting. ```{python} def last_character(word): return word[-1] animals = ["Dog", "Cat", "Bird", "Horse"] animals.sort(key=last_character) print(animals) ``` ::: {.callout-note} ## Exercise: Sort by length Sort the podcast titles from shortest to longest. Use `len` as the sorting key. ```python podcasts = ["Radiolab", "Today Explained", "Serial", "99% Invisible"] ``` ::: ::: {.callout-tip collapse="true"} ## Solution ```python podcasts = ["Radiolab", "Today Explained", "Serial", "99% Invisible"] podcasts.sort(key=len) print(podcasts) ``` ::: ::: {.callout-note} ## Exercise: Fix the program The programmer wants to add two destinations separately, but the result is not correct. Fix one line. ```python destinations = ["Lisbon", "Seoul"] destinations.append(["Cairo", "Oslo"]) print(destinations) ``` ::: ::: {.callout-tip collapse="true"} ## Solution Use `extend()` so that both destinations become individual items. ```python destinations = ["Lisbon", "Seoul"] destinations.extend(["Cairo", "Oslo"]) print(destinations) ``` ::: ## Challenge: Reading tracker Complete a small reading tracker that: 1. Starts with three book titles. 2. Adds a fourth title. 3. Removes one completed book and stores its title in `finished_book`. 4. Sorts the remaining titles alphabetically. 5. Prints the finished book, the updated list, and the number of books still waiting. ::: {.callout-tip collapse="true"} ## Possible solution ```python reading_list = ["Beloved", "Dune", "The Hobbit"] reading_list.append("Pachinko") finished_book = reading_list.pop(1) reading_list.sort() print(f"Finished: {finished_book}") print(f"Still to read: {reading_list}") print(f"Books remaining: {len(reading_list)}") ``` ::: ## Summary - Lists use square brackets: `[]`. - Lists are ordered and mutable. - Lists allow duplicate items. - Indexes and slices retrieve items. - Methods such as `append()`, `remove()`, and `sort()` modify a list in place.