Python List Sorting: From 0 to Infinity

1. What is Sorting?

Sorting means arranging values in a fixed order. For numbers, this usually means smallest to largest. For words, this usually means alphabetical order.

Unsorted list → Compare values → Arrange values → Sorted list

marks = [55, 12, 90, 33, 71]

# After sorting:
# [12, 33, 55, 71, 90]

Ascending Order

Small to big.

[2, 5, 9, 12]

Descending Order

Big to small.

[12, 9, 5, 2]

Alphabetical Order

Words arranged from A to Z.

["apple", "banana", "mango"]

2. Two Main Ways to Sort Lists

Method	Meaning	Changes Original List?	Returns
`list.sort()`	Sorts the same list directly.	Yes	`None`
`sorted(list)`	Creates a new sorted list.	No	New sorted list

Important: Use sort() when you are okay with changing the original list. Use sorted() when you want to keep the original list safe.

3. Sorting with `sort()`

The sort() method sorts the original list itself. This is called in-place sorting.

numbers = [40, 10, 30, 20]

numbers.sort()

print(numbers)

Output:

[10, 20, 30, 40]

Why does `sort()` return `None`?

Because sort() changes the original list directly. It does not create a new list.

numbers = [4, 2, 9, 1]

answer = numbers.sort()

print(numbers)
print(answer)

Output:

[1, 2, 4, 9]
None

Do not write numbers = numbers.sort(). That will make numbers become None.

4. Sorting with `sorted()`

The sorted() function creates a new sorted list. It does not damage the original list.

numbers = [40, 10, 30, 20]

new_numbers = sorted(numbers)

print("Original:", numbers)
print("Sorted:", new_numbers)

Output:

Original: [40, 10, 30, 20]
Sorted: [10, 20, 30, 40]

Rule: sorted() is safer when you want to preserve original data.

5. Sorting in Reverse Order

To sort from largest to smallest, use reverse=True.

marks = [55, 90, 12, 71, 33]

marks.sort(reverse=True)

print(marks)

Output:

[90, 71, 55, 33, 12]

Using `sorted()` with reverse

marks = [55, 90, 12, 71, 33]

top_marks = sorted(marks, reverse=True)

print("Original:", marks)
print("Sorted:", top_marks)

Output:

Original: [55, 90, 12, 71, 33]
Sorted: [90, 71, 55, 33, 12]

6. Sorting Strings

Python can sort strings alphabetically.

names = ["Ravi", "Amit", "Champak", "Zoya"]

names.sort()

print(names)

Output:

['Amit', 'Champak', 'Ravi', 'Zoya']

Capital and Small Letters Problem

Python sorts capital letters before small letters because it uses Unicode values.

words = ["banana", "Apple", "mango", "Cherry"]

print(sorted(words))

Output:

['Apple', 'Cherry', 'banana', 'mango']

To sort without worrying about capital and small letters, use key=str.lower.

words = ["banana", "Apple", "mango", "Cherry"]

print(sorted(words, key=str.lower))

Output:

['Apple', 'banana', 'Cherry', 'mango']

7. Sorting with `key`

The key parameter tells Python: "Sort according to this rule."

Sort words by length

words = ["elephant", "cat", "tiger", "ant"]

words.sort(key=len)

print(words)

Output:

['cat', 'ant', 'tiger', 'elephant']

Here Python does not sort alphabetically. It sorts by the result of len(word).

Word	Length
elephant	8
cat	3
tiger	5
ant	3

8. Sorting with `lambda`

A lambda function is a small one-line function. It is very useful for custom sorting.

Sort tuples by second value

students = [
    ("Amit", 70),
    ("Ravi", 90),
    ("Zoya", 85),
    ("Meena", 60)
]

students.sort(key=lambda student: student[1])

print(students)

Output:

[('Meena', 60), ('Amit', 70), ('Zoya', 85), ('Ravi', 90)]

Here student[1] means marks. So Python sorts students according to marks.

Sort by marks in descending order

students = [
    ("Amit", 70),
    ("Ravi", 90),
    ("Zoya", 85),
    ("Meena", 60)
]

students.sort(key=lambda student: student[1], reverse=True)

print(students)

Output:

[('Ravi', 90), ('Zoya', 85), ('Amit', 70), ('Meena', 60)]

9. Sorting by a Comparison Function

A comparison function compares two values at a time. It tells Python whether the first value should come before the second value, after the second value, or whether both are equal for sorting.

In modern Python 3, sort() and sorted() do not directly accept a comparison function. Instead, we use functools.cmp_to_key().

Full form: cmp means comparison.
cmp_to_key() converts a comparison function into a key function.

How a comparison function works

Return Value	Meaning
`negative number`	First value should come before second value.
`0`	Both values are equal for sorting.
`positive number`	First value should come after second value.

Example 1: Sort numbers using a comparison function

from functools import cmp_to_key

def compare(a, b):
    return a - b

numbers = [40, 10, 30, 20]

numbers.sort(key=cmp_to_key(compare))

print(numbers)

Output:

[10, 20, 30, 40]

Here compare(a, b) returns a - b. If a is smaller, the answer is negative, so a comes before b.

Example 2: Descending order using a comparison function

from functools import cmp_to_key

def compare(a, b):
    return b - a

numbers = [40, 10, 30, 20]

numbers.sort(key=cmp_to_key(compare))

print(numbers)

Output:

[40, 30, 20, 10]

This time we return b - a. So bigger numbers come first.

Example 3: Write comparison clearly using if-else

from functools import cmp_to_key

def compare(a, b):
    if a < b:
        return -1
    elif a > b:
        return 1
    else:
        return 0

numbers = [5, 2, 9, 1]

answer = sorted(numbers, key=cmp_to_key(compare))

print(answer)

Output:

[1, 2, 5, 9]

This style is useful for teaching because it clearly shows the three possible comparison results: before, equal, and after.

Example 4: Sort strings by length using comparison function

from functools import cmp_to_key

def compare_by_length(a, b):
    return len(a) - len(b)

words = ["elephant", "cat", "tiger", "ant"]

words.sort(key=cmp_to_key(compare_by_length))

print(words)

Output:

['cat', 'ant', 'tiger', 'elephant']

The words are sorted according to their lengths.

Example 5: If length is same, sort alphabetically

from functools import cmp_to_key

def compare_words(a, b):
    if len(a) != len(b):
        return len(a) - len(b)

    if a < b:
        return -1
    elif a > b:
        return 1
    else:
        return 0

words = ["dog", "cat", "elephant", "ant", "tiger"]

words.sort(key=cmp_to_key(compare_words))

print(words)

Output:

['ant', 'cat', 'dog', 'tiger', 'elephant']

First Python compares length. If two words have the same length, Python compares them alphabetically.

Example 6: Sort by last character

from functools import cmp_to_key

def compare_last_char(a, b):
    if a[-1] < b[-1]:
        return -1
    elif a[-1] > b[-1]:
        return 1
    else:
        return 0

words = ["banana", "apple", "mango", "grapes"]

words.sort(key=cmp_to_key(compare_last_char))

print(words)

Output:

['banana', 'apple', 'mango', 'grapes']

The sorting is based on the last character of each word.

Example 7: Sort students by marks using comparison function

from functools import cmp_to_key

students = [
    {"name": "Amit", "marks": 70},
    {"name": "Ravi", "marks": 90},
    {"name": "Zoya", "marks": 85},
    {"name": "Meena", "marks": 60}
]

def compare_students(a, b):
    return a["marks"] - b["marks"]

students.sort(key=cmp_to_key(compare_students))

print(students)

Output:

[
  {'name': 'Meena', 'marks': 60},
  {'name': 'Amit', 'marks': 70},
  {'name': 'Zoya', 'marks': 85},
  {'name': 'Ravi', 'marks': 90}
]

Example 8: Sort students by highest marks first

from functools import cmp_to_key

students = [
    {"name": "Amit", "marks": 70},
    {"name": "Ravi", "marks": 90},
    {"name": "Zoya", "marks": 85},
    {"name": "Meena", "marks": 60}
]

def compare_students(a, b):
    return b["marks"] - a["marks"]

students.sort(key=cmp_to_key(compare_students))

print(students)

Output:

[
  {'name': 'Ravi', 'marks': 90},
  {'name': 'Zoya', 'marks': 85},
  {'name': 'Amit', 'marks': 70},
  {'name': 'Meena', 'marks': 60}
]

Example 9: Marks high to low, name A to Z if marks are same

from functools import cmp_to_key

students = [
    {"name": "Ravi", "marks": 90},
    {"name": "Amit", "marks": 70},
    {"name": "Zoya", "marks": 90},
    {"name": "Meena", "marks": 70}
]

def compare_students(a, b):
    if a["marks"] != b["marks"]:
        return b["marks"] - a["marks"]

    if a["name"] < b["name"]:
        return -1
    elif a["name"] > b["name"]:
        return 1
    else:
        return 0

students.sort(key=cmp_to_key(compare_students))

for student in students:
    print(student["name"], student["marks"])

Output:

Ravi 90
Zoya 90
Amit 70
Meena 70

This is a powerful example. First the students are sorted by marks from high to low. If marks are equal, names are sorted alphabetically.

Example 10: Sort products by price ascending and rating descending

from functools import cmp_to_key

products = [
    {"name": "Keyboard", "price": 800, "rating": 4.5},
    {"name": "Mouse", "price": 500, "rating": 4.2},
    {"name": "Monitor", "price": 800, "rating": 4.8},
    {"name": "USB Cable", "price": 200, "rating": 4.0}
]

def compare_products(a, b):
    if a["price"] != b["price"]:
        return a["price"] - b["price"]

    if a["rating"] > b["rating"]:
        return -1
    elif a["rating"] < b["rating"]:
        return 1
    else:
        return 0

products.sort(key=cmp_to_key(compare_products))

for product in products:
    print(product["name"], product["price"], product["rating"])

Output:

USB Cable 200 4.0
Mouse 500 4.2
Monitor 800 4.8
Keyboard 800 4.5

Comparison Function vs Key Function

Feature	Key Function	Comparison Function
Main idea	Give one value for each item.	Compare two items at a time.
Used directly in Python 3?	Yes	No, needs `cmp_to_key()`.
Usually faster?	Yes	Usually slower.
Best for	Simple sorting rules.	Complex custom comparison logic.

Same sorting using key function

students = [
    {"name": "Ravi", "marks": 90},
    {"name": "Amit", "marks": 70},
    {"name": "Zoya", "marks": 90},
    {"name": "Meena", "marks": 70}
]

students.sort(key=lambda student: (-student["marks"], student["name"]))

for student in students:
    print(student["name"], student["marks"])

Output:

Ravi 90
Zoya 90
Amit 70
Meena 70

In most real Python programs, prefer key. Use a comparison function only when the sorting rule is easier to express by comparing two items directly.

Cheat Sheet for Comparison Function

from functools import cmp_to_key

def compare(a, b):
    if a < b:
        return -1
    elif a > b:
        return 1
    else:
        return 0

items.sort(key=cmp_to_key(compare))

10. Sorting List of Dictionaries

Real data often comes as dictionaries. For example, each student can be a dictionary.

students = [
    {"name": "Amit", "marks": 70},
    {"name": "Ravi", "marks": 90},
    {"name": "Zoya", "marks": 85},
    {"name": "Meena", "marks": 60}
]

students.sort(key=lambda student: student["marks"])

print(students)

Output:

[
  {'name': 'Meena', 'marks': 60},
  {'name': 'Amit', 'marks': 70},
  {'name': 'Zoya', 'marks': 85},
  {'name': 'Ravi', 'marks': 90}
]

Sort by name

students.sort(key=lambda student: student["name"])

print(students)

Sort by marks from highest to lowest

students.sort(key=lambda student: student["marks"], reverse=True)

print(students)

11. Sorting by Multiple Rules

Sometimes one rule is not enough. Suppose two students have the same marks. Then we may want to sort by marks first and name second.

students = [
    ("Ravi", 90),
    ("Amit", 70),
    ("Zoya", 90),
    ("Meena", 70)
]

students.sort(key=lambda student: (student[1], student[0]))

print(students)

Output:

[('Amit', 70), ('Meena', 70), ('Ravi', 90), ('Zoya', 90)]

Python first sorts by student[1], which is marks. If marks are equal, it sorts by student[0], which is name.

Marks highest first, name alphabetically second

students = [
    ("Ravi", 90),
    ("Amit", 70),
    ("Zoya", 90),
    ("Meena", 70)
]

students.sort(key=lambda student: (-student[1], student[0]))

print(students)

Output:

[('Ravi', 90), ('Zoya', 90), ('Amit', 70), ('Meena', 70)]

The minus sign before student[1] makes marks sort in descending order. Names still sort in ascending order.

12. Sorting Objects

In Object Oriented Programming, we may have a list of objects. We can sort objects using their attributes.

class Student:
    def __init__(self, name, marks):
        self.name = name
        self.marks = marks

    def __repr__(self):
        return f"{self.name}: {self.marks}"


students = [
    Student("Amit", 70),
    Student("Ravi", 90),
    Student("Zoya", 85),
    Student("Meena", 60)
]

students.sort(key=lambda student: student.marks)

print(students)

Output:

[Meena: 60, Amit: 70, Zoya: 85, Ravi: 90]

Sort objects by name

students.sort(key=lambda student: student.name)

print(students)

Sort objects using comparison function

from functools import cmp_to_key

class Student:
    def __init__(self, name, marks):
        self.name = name
        self.marks = marks

    def __repr__(self):
        return f"{self.name}: {self.marks}"


students = [
    Student("Amit", 70),
    Student("Ravi", 90),
    Student("Zoya", 85),
    Student("Meena", 60)
]

def compare_students(a, b):
    return b.marks - a.marks

students.sort(key=cmp_to_key(compare_students))

print(students)

Output:

[Ravi: 90, Zoya: 85, Amit: 70, Meena: 60]

13. Common Mistakes

Mistake	Problem	Correct Way
`numbers = numbers.sort()`	`sort()` returns `None`.	`numbers.sort()`
Forgetting `reverse=True`	List sorts smallest to largest.	`numbers.sort(reverse=True)`
Sorting mixed types	Python cannot compare numbers and strings directly.	Convert all values to same type first.
Wrong dictionary key	`KeyError`	Use the exact key name.
Using comparison function directly	Python 3 does not accept old-style comparison functions directly.	Use `cmp_to_key(compare)`.

Example of mixed type problem

items = [10, "5", 2]

# This will give an error:
# items.sort()

Fix

items = [10, "5", 2]

items.sort(key=int)

print(items)

Output:

[2, '5', 10]

14. Sorting Without Built-in Functions

Beginners should also understand how sorting works internally. Here is a simple bubble sort example.

numbers = [5, 2, 9, 1]

n = len(numbers)

for i in range(n):
    for j in range(0, n - 1):
        if numbers[j] > numbers[j + 1]:
            numbers[j], numbers[j + 1] = numbers[j + 1], numbers[j]

print(numbers)

Output:

[1, 2, 5, 9]

In real Python programs, use sort() or sorted(). But learning manual sorting helps you understand comparison, loops, and swapping.

15. Full Revision Program

from functools import cmp_to_key

numbers = [45, 12, 90, 33, 7]

print("Original:", numbers)

print("Using sorted:", sorted(numbers))
print("Original still same:", numbers)

numbers.sort()
print("After sort:", numbers)

numbers.sort(reverse=True)
print("Descending:", numbers)


names = ["banana", "Apple", "mango", "Cherry"]

print("Normal string sort:", sorted(names))
print("Case-insensitive sort:", sorted(names, key=str.lower))


students = [
    {"name": "Amit", "marks": 70},
    {"name": "Ravi", "marks": 90},
    {"name": "Zoya", "marks": 85},
    {"name": "Meena", "marks": 60}
]

print("Students by marks:")
print(sorted(students, key=lambda s: s["marks"]))

print("Students by marks high to low:")
print(sorted(students, key=lambda s: s["marks"], reverse=True))


def compare_students(a, b):
    if a["marks"] != b["marks"]:
        return b["marks"] - a["marks"]

    if a["name"] < b["name"]:
        return -1
    elif a["name"] > b["name"]:
        return 1
    else:
        return 0


print("Students using comparison function:")
answer = sorted(students, key=cmp_to_key(compare_students))
print(answer)

16. Practice Questions

Beginner Level

Create a list of 5 numbers and sort it in ascending order.
Create a list of 5 numbers and sort it in descending order.
Create a list of names and sort it alphabetically.
Use sorted() and prove that the original list does not change.

Intermediate Level

Sort words by their length.
Sort names without caring about capital and small letters.
Sort a list of tuples by the second value.
Sort students by marks from highest to lowest.

Comparison Function Practice

Sort numbers in ascending order using a comparison function.
Sort numbers in descending order using a comparison function.
Sort words by length using a comparison function.
Sort words by last character using a comparison function.
Sort students by marks descending and name ascending.
Sort products by price ascending and rating descending.

Advanced Level

Sort a list of dictionaries by price.
Sort products by rating first and price second.
Create a class Book with title and price. Sort books by price.
Write bubble sort manually for a list of numbers.

17. Mini Project: Leaderboard Sorting

Let us create a small leaderboard. We want the highest score first. If two players have the same score, we sort names alphabetically.

Using key function

players = [
    {"name": "Amit", "score": 120},
    {"name": "Ravi", "score": 300},
    {"name": "Zoya", "score": 250},
    {"name": "Meena", "score": 300}
]

leaderboard = sorted(
    players,
    key=lambda player: (-player["score"], player["name"])
)

for rank, player in enumerate(leaderboard, start=1):
    print(rank, player["name"], player["score"])

Output:

1 Meena 300
2 Ravi 300
3 Zoya 250
4 Amit 120

Using comparison function

from functools import cmp_to_key

players = [
    {"name": "Amit", "score": 120},
    {"name": "Ravi", "score": 300},
    {"name": "Zoya", "score": 250},
    {"name": "Meena", "score": 300}
]

def compare_players(a, b):
    if a["score"] != b["score"]:
        return b["score"] - a["score"]

    if a["name"] < b["name"]:
        return -1
    elif a["name"] > b["name"]:
        return 1
    else:
        return 0

leaderboard = sorted(players, key=cmp_to_key(compare_players))

for rank, player in enumerate(leaderboard, start=1):
    print(rank, player["name"], player["score"])

Output:

1 Meena 300
2 Ravi 300
3 Zoya 250
4 Amit 120

18. Final Cheat Sheet

Task	Code
Sort original list	`numbers.sort()`
Create new sorted list	`new_list = sorted(numbers)`
Descending order	`numbers.sort(reverse=True)`
Sort by length	`words.sort(key=len)`
Case-insensitive sort	`sorted(words, key=str.lower)`
Sort tuple by second item	`items.sort(key=lambda x: x[1])`
Sort dictionary by marks	`students.sort(key=lambda s: s["marks"])`
Sort object by attribute	`students.sort(key=lambda s: s.marks)`
Use comparison function	`items.sort(key=cmp_to_key(compare))`

Comparison Function Template

from functools import cmp_to_key

def compare(a, b):
    if a < b:
        return -1
    elif a > b:
        return 1
    else:
        return 0

items.sort(key=cmp_to_key(compare))

19. Practice in the Python Editor

Try the examples above in the embedded Python editor. Change the numbers, names, marks, and sorting rules.

1. What is Sorting?

Ascending Order

Descending Order

Alphabetical Order

2. Two Main Ways to Sort Lists

3. Sorting with sort()

Why does sort() return None?

4. Sorting with sorted()

5. Sorting in Reverse Order

Using sorted() with reverse

6. Sorting Strings

Capital and Small Letters Problem

7. Sorting with key

Sort words by length

8. Sorting with lambda

Sort tuples by second value

Sort by marks in descending order

9. Sorting by a Comparison Function

How a comparison function works

Example 1: Sort numbers using a comparison function

Example 2: Descending order using a comparison function

Example 3: Write comparison clearly using if-else

Example 4: Sort strings by length using comparison function

Example 5: If length is same, sort alphabetically

Example 6: Sort by last character

Example 7: Sort students by marks using comparison function

Example 8: Sort students by highest marks first

Example 9: Marks high to low, name A to Z if marks are same

Example 10: Sort products by price ascending and rating descending

Comparison Function vs Key Function

Same sorting using key function

Cheat Sheet for Comparison Function

10. Sorting List of Dictionaries

Sort by name

Sort by marks from highest to lowest

11. Sorting by Multiple Rules

Marks highest first, name alphabetically second

12. Sorting Objects

Sort objects by name

Sort objects using comparison function

13. Common Mistakes

Example of mixed type problem

Fix

14. Sorting Without Built-in Functions

15. Full Revision Program

16. Practice Questions

Beginner Level

Intermediate Level

Comparison Function Practice

Advanced Level

17. Mini Project: Leaderboard Sorting

Using key function

Using comparison function

18. Final Cheat Sheet

Comparison Function Template

19. Practice in the Python Editor

3. Sorting with `sort()`

Why does `sort()` return `None`?

4. Sorting with `sorted()`

Using `sorted()` with reverse

7. Sorting with `key`

8. Sorting with `lambda`