A list in Python holds an ordered collection of items. Its defined with square brackets, and the items are separated by commas.

numbers = [10, 20, 30, 40]
names = ["alice", "bob", "carol"]

Accessing Items

By Index

fruits = ["apple", "banana", "cherry"]
print(fruits[0])  # apple
print(fruits[-1]) # cherry

Using Methods

index(item, start, end)

Returns the first index of the item. Raises ValueError if the item is absent.

colors = ["red", "green", "blue", "green"]
print(colors.index("green"))      # 1
# print(colors.index("yellow"))  # ValueError

count(item)

Returns how many times an item appears. Returns 0 if the item does not exist.

colors = ["red", "green", "blue", "green"]
print(colors.count("green"))   # 2
print(colors.count("yellow"))  # 0

len(list)

Returns the total number of items in the list.

tools = ["hammer", "screwdriver", "wrench"]
print(len(tools))  # 3

Membership Tests

in

Returns True if the item is present.

devices = ["phone", "tablet", "laptop"]
print("tablet" in devices)   # True
print("watch" in devices)    # False

not in

Returns True if the item is absent.

devices = ["phone", "tablet", "laptop"]
print("watch" not in devices)  # True

Adding Items

append(item)

Adds an item to the end of the list.

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

extend(iterable)

Extends the list by appending each element from the iterable. When extending with a string, every character becomes a separate element.

chars = ["x", "y"]
chars.extend("ab")
print(chars)  # ['x', 'y', 'a', 'b']

more = ["p", "q"]
more.extend(["r", "s"])
print(more)  # ['p', 'q', 'r', 's']

insert(index, item)

Inserts an item at a specific position. Existing items shift to the right.

grades = ["A", "B", "D"]
grades.insert(2, "C")
print(grades)  # ['A', 'B', 'C', 'D']

Removing Items

del list[index]

Deletes an item at a given index, or the whole list if no index is provided.

values = [100, 200, 300, 400]
del values[1]
print(values)  # [100, 300, 400]

pop(index)

Removes and returns the item at the specified index. The last item is removed if no index is given.

cities = ["Paris", "London", "Tokyo"]
removed = cities.pop(1)
print(removed)  # London
print(cities)   # ['Paris', 'Tokyo']

remove(item)

Removes the first occurrence of the item. A ValueError is raised if the item is not found.

letters = ["a", "b", "c", "b"]
letters.remove("b")
print(letters)  # ['a', 'c', 'b']

clear()

Empties the list, leaving [].

data = [1, 2, 3]
data.clear()
print(data)  # []

Modifying Items

Direct assignment

shapes = ["circle", "square", "triangle"]
shapes[1] = "rectangle"
print(shapes)  # ['circle', 'rectangle', 'triangle']

reverse()

Reverses the order of the items in place.

nums = [1, 3, 5, 7]
nums.reverse()
print(nums)  # [7, 5, 3, 1]

sort(key=None, reverse=False)

Sorts the list in place. Set reverse=True for descending order.

scores = [88, 92, 75, 83]
scores.sort()
print(scores)  # [75, 83, 88, 92]
scores.sort(reverse=True)
print(scores)  # [92, 88, 83, 75]

Copying a List

copy()

Creates a shallow copy of the list.

original = [10, 20, 30]
duplicate = original.copy()
print(duplicate)  # [10, 20, 30]
# Changing the copy does not affect the original
duplicate[0] = 99
print(original)   # [10, 20, 30]

Iterating Over a List

while loop

tasks = ["write", "test", "deploy"]
index = 0
while index < len(tasks):
    print(tasks[index])
    index += 1

for loop

tasks = ["write", "test", "deploy"]
for task in tasks:
    print(task)

Nested Lists

A list can contain other lists, forming a 2D (or higher) structure.

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

print(matrix[0])      # [1, 2, 3]
print(matrix[1][2])   # 6

mixed = ["text", 42, [True, None]]
print(mixed[2][0])    # True