Python Nested Lists

Let’s Talk About Lists Inside Lists

We know what a list is. It is a collection of items.

numbers = [1, 2, 3, 4, 5]
names = ["Alex", "Sam", "John"]

But what if we want to store a list inside another list?

That is a nested list. A list that contains other lists as its items.

Think of it like a table. A table has rows and columns. Each row is a list. And the whole table is a list of rows.

That is a nested list.

How to Create a Nested List in Python

We just put lists inside a list.

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

That is a 3x3 matrix. Three rows. Three columns.

It is a list. And each item in that list is another list.

We can also create smaller ones.

pairs = [[1, 2], [3, 4], [5, 6]]

Or mixed ones.

data = [["Alex", 25], ["Sam", 30], ["John", 22]]

That is a list of name-age pairs.

Accessing Items in a Nested List

To access an item in a nested list, we use two indices.

• First index: which inner list we want
• Second index: which item in that inner list

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

print(matrix[0])

[1, 2, 3]

That gives us the first inner list.

print(matrix[0][1])

2

That gives us the second item of the first inner list.

Let me show more.

print(matrix[1][2])   # 6
print(matrix[2][0])   # 7
print(matrix[-1][-1]) # 9

Let’s remember: first index for the row, second index for the column.

Changing Items in a Nested List

We can change items the same way. Just use both indices.

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

matrix[0][1] = 20
print(matrix)

[[1, 20, 3], [4, 5, 6], [7, 8, 9]]

We changed the item at row 0, column 1. It was 2. Now it is 20.

Looping Through a Nested List in Python

We can use nested loops to go through all items.

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

for row in matrix:
    for num in row:
        print(num, end=" ")
    print()

1 2 3
4 5 6
7 8 9

The outer loop goes through each row. The inner loop goes through each item in that row.

We can also use index-based loops.

for i in range(len(matrix)):
    for j in range(len(matrix[i])):
        print(matrix[i][j], end=" ")
    print()

Same output. But the first way is cleaner.

Python Nested Lists: A Real-Life Example — A Grade Book

Let’s say we have a list of students. Each student has a name and a list of scores.

students = [
    ["Alex", [85, 90, 78]],
    ["Sam", [92, 88, 95]],
    ["John", [70, 65, 80]]
]

for student in students:
    name = student[0]
    scores = student[1]
    average = sum(scores) / len(scores)
    print(f"{name}: Average = {average:.1f}")

Alex: Average = 84.3
Sam: Average = 91.7
John: Average = 71.7

That is a natural use of nested lists. Each student is a list. And their scores are another list inside.

A Real-Life Example — A Game Board

Let’s create a tic-tac-toe board.

board = [
    [" ", " ", " "],
    [" ", " ", " "],
    [" ", " ", " "]
]

def print_board():
    for row in board:
        print("|".join(row))
        print("-" * 5)

print_board()

 | |
-----
 | |
-----
 | |
-----

Then we can place marks.

board[1][1] = "X"
board[0][0] = "O"
print_board()

O| |
-----
 |X|
-----
 | |
-----

That is how we can use nested lists to represent a board.

Flattening a Nested List in Python

Sometimes we want to turn a nested list into a single list. That is called flattening.

We can use a list comprehension.

matrix = [[1, 2], [3, 4], [5, 6]]
flattened = [num for row in matrix for num in row]
print(flattened)

[1, 2, 3, 4, 5, 6]

Or a loop.

flattened = []
for row in matrix:
    for num in row:
        flattened.append(num)
print(flattened)

Both work. The comprehension is shorter.

Creating a Nested List in Python

Let’s be careful when creating nested lists with repetition.

matrix = [[0] * 3] * 3
print(matrix)

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

Looks fine. But if we change one item, all rows change.

matrix[0][0] = 1
print(matrix)

[[1, 0, 0], [1, 0, 0], [1, 0, 0]]

Why? Because [[0] * 3] * 3 creates three references to the same inner list. So changing one changes all.

To avoid this, we use a list comprehension.

matrix = [[0] * 3 for _ in range(3)]
matrix[0][0] = 1
print(matrix)

[[1, 0, 0], [0, 0, 0], [0, 0, 0]]

Now each row is a separate list. That is the safe way.

A Real-Life Example — Seating Chart

seats = [
    ["A1", "A2", "A3"],
    ["B1", "B2", "B3"],
    ["C1", "C2", "C3"]
]

for row in seats:
    print(" ".join(row))

A1 A2 A3
B1 B2 B3
C1 C2 C3

That is a simple seating chart.

Common Mistakes with Python Nested Lists

matrix = [[1, 2], [3, 4]]
print(matrix[0])    # [1, 2]
print(matrix[0][0]) # 1

matrix = [[1, 2], [3, 4]]
print(matrix[2][0])   # Error!

matrix = [[1, 2], [3, 4]]
print(matrix[0][1])   # 2
print(matrix[1][0])   # 3

matrix = [[0] * 3] * 3
matrix[0][0] = 1
print(matrix)

[[1, 0, 0], [1, 0, 0], [1, 0, 0]]

matrix = [[0] * 3 for _ in range(3)]