Lists

A list holds many values in order: a shopping list, the marks of a class, the messages in a chat. It is the collection you will use most in Python, so this lesson covers it properly – including the copying trap that catches nearly every beginner.

Creating a list

marks = [78, 92, 65, 88]
names = ["Asha", "Ben", "Chitra"]
mixed = [1, "two", 3.0, True, None]   # allowed, but usually a bad idea
empty = []

print(marks)
print(len(names), len(empty))

[78, 92, 65, 88]
3 0

Lists use square brackets and commas. Indexing and slicing work exactly like strings:

marks = [78, 92, 65, 88]
print(marks[0], marks[-1])
print(marks[1:3])
print(marks[::-1])

78 88
[92, 65]
[88, 65, 92, 78]

Lists are mutable

Unlike strings, you can change a list in place:

tasks = ["email", "report", "call"]
tasks[1] = "presentation"
print(tasks)

['email', 'presentation', 'call']

Adding items

cart = ["rice"]
cart.append("dal")                 # one item at the end
cart.insert(0, "oil")              # at a position
cart.extend(["salt", "sugar"])     # many items at the end
print(cart)

['oil', 'rice', 'dal', 'salt', 'sugar']

Removing items

queue = ["Arun", "Bela", "Chris", "Bela", "Dev"]

queue.remove("Bela")       # first matching value
print(queue)
last = queue.pop()         # remove and return the last item
first = queue.pop(0)       # remove and return by index
print(first, last, queue)
del queue[0]               # delete by index
print(queue)
queue.clear()
print(queue)

['Arun', 'Chris', 'Bela', 'Dev']
Arun Dev ['Chris', 'Bela']
['Bela']
[]

Searching and counting

rolls = [4, 6, 2, 6, 1, 6]
print(6 in rolls)
print(rolls.count(6))
print(rolls.index(2))
print(min(rolls), max(rolls), sum(rolls))

True
3
2
1 6 25

Sorting: sort() or sorted()?

list.sort() sorts the list in place and returns None. sorted(any_iterable) leaves the original alone and returns a new list.

scores = [70, 95, 82, 60]
ranked = sorted(scores, reverse=True)
print(ranked, scores)     # original unchanged

scores.sort()
print(scores)

result = scores.sort()
print(result)             # sort() returns None!

[95, 82, 70, 60] [70, 95, 82, 60]
[60, 70, 82, 95]
None

Sorting with a key

Pass key= a function that says what to sort by:

words = ["banana", "Kiwi", "apple", "fig"]
print(sorted(words))                  # capitals first
print(sorted(words, key=str.lower))   # alphabetical, ignoring case
print(sorted(words, key=len))         # by length

students = [("Ria", 88), ("Om", 95), ("Lata", 72)]
print(sorted(students, key=lambda s: s[1], reverse=True))

['Kiwi', 'apple', 'banana', 'fig']
['apple', 'banana', 'fig', 'Kiwi']
['fig', 'Kiwi', 'apple', 'banana']
[('Om', 95), ('Ria', 88), ('Lata', 72)]

lambda s: s[1] is a tiny unnamed function meaning "give me the second element". You will meet lambdas properly in chapter 9.

Copying a list

Remember from the Variables lesson: b = a does not copy. Both names point at the same list.

original = [1, 2, 3]
alias = original
copy1 = original.copy()
copy2 = original[:]
copy3 = list(original)

original.append(4)
print(alias)
print(copy1, copy2, copy3)

[1, 2, 3, 4]
[1, 2, 3] [1, 2, 3] [1, 2, 3]

A shallow copy duplicates the outer list only. If the items are themselves lists, they are still shared:

import copy

grid = [[0, 0], [0, 0]]
shallow = grid.copy()
deep = copy.deepcopy(grid)

grid[0][0] = 9
print(shallow)   # inner list is shared
print(deep)      # fully independent

[[9, 0], [0, 0]]
[[0, 0], [0, 0]]

Lists of lists (2-D data)

sales = [
    ["Jan", 120, 90],
    ["Feb", 150, 110],
    ["Mar", 170, 95],
]
for month, online, store in sales:
    print(f"{month}: total {online + store}")
print("Feb online:", sales[1][1])

Jan: total 210
Feb: total 260
Mar: total 265
Feb online: 150

Practice

Summary