← Back to Tutorials Chapter 5

Lists

List Basics

A list is an ordered, mutable collection of items enclosed in square brackets []:

fruits = ["apple", "banana", "cherry"]
mixed = [1, "hello", 3.14, True]
empty = []
numbers = list(range(5))  # [0, 1, 2, 3, 4]

Indexing and Slicing

items = ["a", "b", "c", "d", "e"]
items[0]     # "a"
items[-1]    # "e"
items[1:3]   # ["b", "c"]
items[:3]    # ["a", "b", "c"]
items[::2]   # ["a", "c", "e"]
items[::-1]  # ["e", "d", "c", "b", "a"]

Mutability

Unlike strings, lists can be modified in place:

nums = [1, 2, 3]
nums[0] = 99
print(nums)  # [99, 2, 3]

List Methods

lst = [1, 2, 3]

lst.append(4)          # [1, 2, 3, 4]
lst.extend([5, 6])     # [1, 2, 3, 4, 5, 6]
lst.insert(0, 0)       # [0, 1, 2, 3, 4, 5, 6]
lst.remove(3)          # removes first 3
popped = lst.pop()     # removes and returns last item
lst.pop(0)             # removes and returns index 0
lst.clear()            # []

lst = [3, 1, 4, 1, 5]
lst.index(4)           # 2
lst.count(1)           # 2
lst.sort()             # [1, 1, 3, 4, 5]
lst.sort(reverse=True) # [5, 4, 3, 1, 1]
lst.reverse()          # reverse in place
copy = lst.copy()      # shallow copy

List Comprehension

A concise way to create lists:

squares = [x ** 2 for x in range(10)]
evens = [x for x in range(20) if x % 2 == 0]
pairs = [(a, b) for a in [1,2] for b in ["x","y"]]
# [(1, 'x'), (1, 'y'), (2, 'x'), (2, 'y')]

Looping Over Lists

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

for color in colors:
    print(color)

for i, color in enumerate(colors):
    print(i, color)

for a, b in zip([1,2], ["x","y"]):
    print(a, b)

Sorting

words = ["banana", "apple", "Cherry"]
words.sort(key=str.lower)  # case-insensitive sort

students = [("Alice", 85), ("Bob", 92), ("Eve", 78)]
students.sort(key=lambda s: s[1], reverse=True)
# [("Bob", 92), ("Alice", 85), ("Eve", 78)]

Copying: Shallow vs Deep

import copy

original = [[1, 2], [3, 4]]
shallow = copy.copy(original)
shallow[0][0] = 99     # affects original too!

deep = copy.deepcopy(original)
deep[0][0] = 100       # original unchanged

Nested Lists

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

# Flatten a nested list
flat = [item for row in matrix for item in row]
# [1, 2, 3, 4, 5, 6, 7, 8, 9]