Lists

So far, each variable has held a single value. But programs often deal with many values: a shopping list, a class of students, a series of temperatures. A list holds any number of values in order, in a single variable.

Creating a list

Write the items inside square brackets, separated by commas:

fruits = ["apple", "banana", "cherry"]
scores = [88, 92, 75, 64]
mixed = ["Ada", 36, True, 1.75]
empty = []

print(fruits)
print(scores)
print(mixed)
print(empty)
print(type(fruits))

A list can hold any types, even a mix. In practice, most lists hold items of the same kind. The items stay in the order you wrote them, and the same value can appear more than once.

len() tells you how many items a list has:

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

Indexing and slicing

Lists use the same indexes and slices as strings. The first item is at index 0, and negative indexes count from the end:

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

print(fruits[0])      # first
print(fruits[2])
print(fruits[-1])     # last
print(fruits[1:3])    # a new list
print(fruits[:2])
print(fruits[::-1])   # reversed

The difference to notice: an index gives you one item (fruits[0] is the string 'apple'), while a slice always gives you a new list, even if it holds only one item (fruits[0:1] is ['apple']).

As with strings, an index past the end raises an IndexError. A list of 5 items has indexes 0 to 4.

Lists can change

Unlike strings, lists are mutable: you can change them after they're created. Assign to an index to replace an item:

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

colors[1] = "yellow" replaces the item at index 1; the list keeps the same length. You can only replace items that already exist: colors[3] = "pink" on this 3-item list is an IndexError. To add a new item, use the methods in the next section.

Adding items

tasks = ["email Bob", "buy milk"]

# add one item to the end
tasks.append("call mum")
print(tasks)

# add at a position
tasks.insert(0, "wake up")
print(tasks)

# add several items to the end
tasks.extend(["cook", "read"])
print(tasks)

These methods change the list itself and give back nothing, so you call them on their own line rather than printing them.

A common mix-up: append() with a list adds the whole list as one item, nested inside. Use extend() to add the items one by one:

a = [1, 2]
a.append([3, 4])
print(a)

b = [1, 2]
b.extend([3, 4])
print(b)

You can also join two lists with +:

morning = ["run", "shower"]
evening = ["cook", "sleep"]
print(morning + evening)

Unlike extend(), + builds a new list and leaves both originals unchanged.

Removing items

pets = ["cat", "dog", "fish", "dog", "hamster"]

# removes the FIRST "dog" only
pets.remove("dog")
print(pets)

# removes the last item and gives it back
last = pets.pop()
print(last, pets)

# removes index 0, gives it back
first = pets.pop(0)
print(first, pets)

# deletes the item at index 0
del pets[0]
print(pets)

pop() is the one to use when you want to remove an item and use it, as with last above.

Removing a value that isn't there raises a ValueError:

pets = ["cat", "dog"]
pets.remove("parrot")

Similarly, pop() on an empty list raises an IndexError. To be safe, check first with in (next section).

Searching a list

numbers = [4, 8, 15, 16, 23, 42, 8]

# is it in the list?
print(15 in numbers)
print(99 not in numbers)

# position of the first match
print(numbers.index(16))

# how many times it appears
print(numbers.count(8))

in and not in answer True or False. index() gives the position of the first match, but raises a ValueError if the value isn't there at all, so check with in first when you're not sure. count() simply gives 0 for a missing value.

Sorting

sort() sorts the list in place, which changes the original. sorted() returns a new sorted list and leaves the original alone.

scores = [64, 92, 75, 88]

scores.sort()
print(scores)

scores.sort(reverse=True)
print(scores)

# sorted() gives back a new list
print(sorted([3, 1, 2]))

sort() changes the list and gives back None (Python's "nothing"). So never write scores = scores.sort(): that sorts the list, then replaces it with None:

scores = [64, 92, 75, 88]
scores = scores.sort()
print(scores)

Use scores.sort() on its own line, or scores = sorted(scores).

Sorting only works when the items can be compared with each other: numbers with numbers, strings with strings. Sorting [3, "two", 1] is a TypeError.

reverse() flips the order in place:

steps = [1, 2, 3, 4]
steps.reverse()
print(steps)

names = ["Zoe", "adam", "Mia"]

print(sorted(names))
print(sorted(names, key=str.lower))
print(names)   # unchanged

Numbers in lists

Python has built-in functions that work on a whole list of numbers:

temperatures = [21.5, 19.0, 24.3, 22.8, 18.1]

print(min(temperatures))
print(max(temperatures))
print(round(sum(temperatures), 1))

# average: total / how many
print(round(sum(temperatures) / len(temperatures), 1))

sum() adds up every item, so it only works on lists of numbers. There's no built-in average(): divide the sum() by the len(), as on the last line.

Copying a list

Assigning a list to another variable does not copy it. Remember the label picture from the Variables lesson: alias = original sticks a second label on the same list. Change the list through either name, and both names see the change:

original = [1, 2, 3]
alias = original
alias.append(4)

print(original)   # changed too!

To make an independent copy, use copy() or a full slice:

original = [1, 2, 3]
copy1 = original.copy()
copy2 = original[:]

copy1.append(4)
print(original)
print(copy1)

grid = [
    [1, 2, 3],
    [4, 5, 6],
    [7, 8, 9],
]

print(grid[0])       # first row
# row 1, column 2: 6
print(grid[1][2])

grid = [[1, 2], [3, 4]]
clone = grid.copy()

clone[0].append(99)
print(grid)    # the original changed too!

import copy

grid = [[1, 2], [3, 4]]
clone = copy.deepcopy(grid)

clone[0].append(99)
print(grid)    # unchanged
print(clone)

Converting to and from lists

# string -> list of characters
print(list("hello"))

# string -> list, split at commas
print("a,b,c".split(","))

# list of strings -> one string
print(", ".join(["x", "y", "z"]))

# the numbers 0 to 4
print(list(range(5)))

range() gets a full explanation in the For Loops lesson.

Exercises

Exercise 1: Shopping list

Start with an empty list. Add "eggs", "bread" and "milk", then remove "bread" and print the list. It should print ['eggs', 'milk'].

shopping = []

Exercise 2: Test results

Print the highest score, the lowest score and the average (rounded to 1 decimal place).

scores = [72, 85, 90, 66, 98, 79]

Exercise 3: Top three

Print the top three scores, highest first: [98, 90, 85]. Don't change the original list.

scores = [72, 85, 90, 66, 98, 79]

Exercise 4: First and last swap

Swap the first and last items of the list, so it prints ['e', 'b', 'c', 'd', 'a'].

letters = ["a", "b", "c", "d", "e"]

print(letters)

List methods at a glance

A quick reference for later. Each example starts from nums = [3, 1, 2]. The Result column shows what the method gives back, or what nums looks like afterwards for methods that change the list.

Built-in functions that work on lists:

Summary

• A list stores many values in order: [1, 2, 3].
• Index and slice lists like strings. len() gives the number of items.
• Unlike strings, lists can be changed: lst[i] = value replaces an item.
• Add items with append, insert and extend. Remove them with remove, pop and del.
• sort() changes the list, while sorted() returns a new one.
• min, max and sum work on lists of numbers.
• b = a doesn't copy a list. Use a.copy() instead.