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.

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 slice is a new list
print(fruits[:2])
print(fruits[::-1])   # reversed copy

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)

Adding items

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

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

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

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

You can also join two lists with +:

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

Removing items

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

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

last = pets.pop()         # removes and returns the last item
print(last, pets)

first = pets.pop(0)       # removes and returns the item at an index
print(first, pets)

del pets[0]               # deletes by index
print(pets)

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

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

Searching a list

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

print(15 in numbers)        # is it in the list?
print(99 not in numbers)
print(numbers.index(16))    # position of the first match
print(numbers.count(8))     # how many times it appears

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)

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

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))
print(round(sum(temperatures) / len(temperatures), 1))   # average

Copying a list

Assigning a list to another variable does not copy it. Both names point to the same list:

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
print(grid[1][2])    # row 1, column 2 -> 6

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

print(list("hello"))               # string -> list of characters
print("a,b,c".split(","))          # string -> list
print(", ".join(["x", "y", "z"]))  # list of strings -> string
print(list(range(5)))              # numbers 0 to 4

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.