Lists: Many Things at Once 📋

A list is things in a row

inventory = ["rubber chicken", "map", "grog", "sword"]

print(inventory)
print(len(inventory))
print(inventory[0])
print(inventory[-1])

['rubber chicken', 'map', 'grog', 'sword']
4
rubber chicken
sword

Square brackets, commas between items. Positions count from zero, negatives count from the right, and slicing works exactly as it did for strings, because both are sequences.

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

print(letters[1:4])
print(letters[:3])
print(letters[-2:])
print(letters[::2])
print(letters[::-1])

['b', 'c', 'd']
['a', 'b', 'c']
['e', 'f']
['a', 'c', 'e']
['f', 'e', 'd', 'c', 'b', 'a']

Unlike strings, lists can be changed

crew = ["Guybrush", "Elaine", "Otis"]

crew[2] = "Meathook"            # replace in place
print(crew)

crew.append("Carla")            # add one to the end
print(crew)

crew.insert(0, "LeChuck")       # add at a position
print(crew)

crew.remove("LeChuck")          # remove by value (the first match)
print(crew)

gone = crew.pop()               # remove the last, and give it back
print(gone, crew)

del crew[0]                     # remove by position
print(crew)

['Guybrush', 'Elaine', 'Meathook']
['Guybrush', 'Elaine', 'Meathook', 'Carla']
['LeChuck', 'Guybrush', 'Elaine', 'Meathook', 'Carla']
['Guybrush', 'Elaine', 'Meathook', 'Carla']
Carla ['Guybrush', 'Elaine', 'Meathook']
['Elaine', 'Meathook']

This is the crucial difference from strings. Strings are immutable: every operation returns a new string. Lists are mutable: operations change the list you already have. Almost every surprise in this lesson comes from that one fact.

The methods worth knowing

scores = [88, 42, 95, 61]

new_list = sorted(scores)
print(scores, "unchanged")
print(new_list, "the sorted copy")

scores.sort()
print(scores, "now sorted in place")

scores.sort(reverse=True)
print(scores, "descending")

[88, 42, 95, 61] unchanged
[42, 61, 88, 95] the sorted copy
[42, 61, 88, 95] now sorted in place
[95, 88, 61, 42] descending

Sorting by something other than the value

crew = ["Guybrush", "Otis", "Elaine", "Meathook"]

print(sorted(crew))                    # alphabetical
print(sorted(crew, key=len))           # by length
print(sorted(crew, key=str.lower))     # case-insensitive
print(max(crew, key=len))

['Elaine', 'Guybrush', 'Meathook', 'Otis']
['Otis', 'Elaine', 'Guybrush', 'Meathook']
['Elaine', 'Guybrush', 'Meathook', 'Otis']
Guybrush

key= takes a function and sorts by what that function returns. It is one of the most useful arguments in the whole language, and it comes back constantly once you have data with structure. Note Guybrush and Meathook are both eight letters, and Python kept them in their original relative order: Python's sort is stable, which is a guarantee you can rely on.

Searching and testing

inventory = ["map", "grog", "sword"]

print("grog" in inventory)
print("banana" in inventory)
print(inventory.index("grog"))
print(inventory.count("grog"))

True
False
1
1

The trap: two names, one list

original = ["a", "b", "c"]
copy = original          # this is NOT a copy

copy.append("d")

print("original:", original)
print("copy:    ", copy)
print("same object?", original is copy)

original: ['a', 'b', 'c', 'd']
copy:     ['a', 'b', 'c', 'd']
same object? True

Three ways to make a genuine copy:

original = ["a", "b", "c"]

copy1 = original.copy()
copy2 = list(original)
copy3 = original[:]

copy1.append("changed")
print(original)
print(copy1)

['a', 'b', 'c']
['a', 'b', 'c', 'changed']

All three are equivalent for a flat list. If your list contains other lists, these are shallow copies: the outer list is new, the inner lists are still shared. For that, copy.deepcopy(), which Lesson 15 covers.

Building a list up

squares = []

for n in range(1, 6):
    squares.append(n * n)

print(squares)
print(sum(squares), min(squares), max(squares))

[1, 4, 9, 16, 25]
55 1 25

Start empty, append in a loop, use afterwards. This is the most common shape in beginner Python, and in Lesson 16 you will learn to write it in one line with a comprehension. Learn it this way first; the short version means nothing if you cannot see the loop inside it.

Lists of anything, including lists

mixed = [42, "grog", 3.5, True, None]
print(mixed)

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

for row in grid:
    print(" ".join(str(n) for n in row))

[42, 'grog', 3.5, True, None]
[4, 5, 6]
6
1 2 3
4 5 6
7 8 9

A list can hold anything, including other lists. grid[1][2] reads as "row 1, then item 2 of that". Mixed-type lists are legal but usually a smell: if the items mean different things, you probably want a dictionary (Lesson 13) or a class (Lesson 31).

inventory = ["sword", "map", "grog"]

inventory.append("rubber chicken")
inventory.append("mints")
inventory.remove("grog")
inventory.sort()

print(f"You are carrying {len(inventory)} things:")
for i, item in enumerate(inventory, start=1):
    print(f"  {i}. {item}")

You are carrying 4 things:
  1. map
  2. mints
  3. rubber chicken
  4. sword

scores = [88, 42, 95, 61, 73]

print(f"Highest: {max(scores)}")
print(f"Lowest:  {min(scores)}")
print(f"Total:   {sum(scores)}")
print(f"Average: {sum(scores) / len(scores):.1f}")

ordered = sorted(scores)
middle = len(ordered) // 2
print(f"Median:  {ordered[middle]}")

Highest: 95
Lowest:  42
Total:   359
Average: 71.8
Median:  73

a = [1, 2, 3]
b = a
c = a.copy()

b.append(4)
c.append(5)

print(a)
print(b)
print(c)