Python Basics — Lists: Practice Exercises

2 minute read

Lists — Practice Exercises

Test your understanding of list creation, methods, slicing, comprehensions, and nested lists.

Exercise 1 — Reverse Without Built-in

Write a function reverse_list(lst) that returns a new list with elements in reverse order. Do not use .reverse(), reversed(), or [::-1].

Show Solution 
def reverse_list(lst):
    result = []
    for i in range(len(lst) - 1, -1, -1):
        result.append(lst[i])
    return result

print(reverse_list([1, 2, 3, 4, 5]))  # [5, 4, 3, 2, 1]
print(reverse_list(["a", "b", "c"]))  # ['c', 'b', 'a']
print(reverse_list([]))               # []

Exercise 2 — Remove Duplicates (Preserve Order)

Write a function remove_duplicates(lst) that returns a new list with duplicates removed, preserving the original order.

Show Solution 
def remove_duplicates(lst):
    seen = set()
    result = []
    for item in lst:
        if item not in seen:
            seen.add(item)
            result.append(item)
    return result

print(remove_duplicates([1, 2, 2, 3, 1, 4, 3]))
# [1, 2, 3, 4]

print(remove_duplicates(["a", "b", "a", "c", "b"]))
# ['a', 'b', 'c']

Exercise 3 — Flatten a Nested List

Write a function flatten(nested) that takes a list of lists and returns a single flat list.

Show Solution 
def flatten(nested):
    return [item for sublist in nested for item in sublist]

print(flatten([[1, 2], [3, 4], [5]]))
# [1, 2, 3, 4, 5]

print(flatten([["a", "b"], ["c"]]))
# ['a', 'b', 'c']

print(flatten([[], [1], [], [2, 3]]))
# [1, 2, 3]

Exercise 4 — List Comprehension Practice

Using a list comprehension, create a list of all numbers from 1 to 50 that are divisible by 3 but not by 5. Then create a list of their squares.

Show Solution 
# Numbers divisible by 3 but not by 5
numbers = [x for x in range(1, 51) if x % 3 == 0 and x % 5 != 0]
print(numbers)
# [3, 6, 9, 12, 18, 21, 24, 27, 33, 36, 39, 42, 48]

# Their squares
squares = [x ** 2 for x in numbers]
print(squares)
# [9, 36, 81, 144, 324, 441, 576, 729, 1089, 1296, 1521, 1764, 2304]

# Or in one step
squares = [x ** 2 for x in range(1, 51) if x % 3 == 0 and x % 5 != 0]
print(squares)

Exercise 5 — Matrix Rotation

Write a function rotate_90(matrix) that rotates a square matrix 90 degrees clockwise.

Hint: Transpose + reverse each row.

Input:          Output:
1 2 3           7 4 1
4 5 6    →      8 5 2
7 8 9           9 6 3
Show Solution 
def rotate_90(matrix):
    n = len(matrix)
    # Transpose then reverse each row
    rotated = [[matrix[n - 1 - j][i] for j in range(n)] for i in range(n)]
    return rotated

# Alternative using zip
def rotate_90_zip(matrix):
    return [list(row) for row in zip(*matrix[::-1])]

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

result = rotate_90(m)
for row in result:
    print(row)
# [7, 4, 1]
# [8, 5, 2]
# [9, 6, 3]

result2 = rotate_90_zip(m)
for row in result2:
    print(row)
# [7, 4, 1]
# [8, 5, 2]
# [9, 6, 3]

Scoring Guide

Score Level
5/5 Excellent — lists mastered
3-4/5 Good — review slicing and comprehensions
1-2/5 Keep practicing — re-read the chapter

Next chapter: Tuples →

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