Python Sort a List – Ascending, Descending, Custom Key

Introduction – Why Learn List Sorting?

Sorting is a fundamental operation in Python programming. Whether you need to sort:

• Numbers
• 🅰️ Strings
• Tuples or objects
• Custom conditions

Python provides powerful and flexible sorting tools using .sort() and sorted().

In this guide, you’ll learn:

• How to sort a list in Python (ascending and descending)
• Difference between sort() and sorted()
• Sort strings, tuples, dictionaries, and objects
• Use custom key functions for advanced sorting
• Best practices for clean, reliable code

1. Sort a List of Numbers – Ascending

nums = [5, 2, 9, 1]
nums.sort()
print(nums)  # [1, 2, 5, 9]

list.sort() sorts the list in place.

2. Sort a List of Numbers – Descending

nums = [5, 2, 9, 1]
nums.sort(reverse=True)
print(nums)  # [9, 5, 2, 1]

Add reverse=True to flip the order.

3. Use sorted() – Returns a New List

nums = [5, 2, 9, 1]
new_nums = sorted(nums)
print(new_nums)   # [1, 2, 5, 9]
print(nums)       # Original remains unchanged

Use sorted() when you don’t want to modify the original list.

4. Sort Strings Alphabetically

words = ["banana", "apple", "cherry"]
words.sort()
print(words)  # ['apple', 'banana', 'cherry']

Case-Insensitive Sort

words = ["Banana", "apple", "Cherry"]
words.sort(key=str.lower)
print(words)  # ['apple', 'Banana', 'Cherry']

Use key=str.lower to ignore case sensitivity.

5. Sort by Length of Words

words = ["a", "apple", "banana"]
words.sort(key=len)
print(words)  # ['a', 'apple', 'banana']

Pass any function to key for custom sort criteria.

6. Sort a List of Tuples by Second Value

items = [(1, 'b'), (3, 'a'), (2, 'c')]
items.sort(key=lambda x: x[1])
print(items)  # [(3, 'a'), (1, 'b'), (2, 'c')]

Use lambda functions for advanced sorting logic.

7. Sort List of Dictionaries by a Key

people = [{"name": "Alice", "age": 30}, {"name": "Bob", "age": 25}]
people.sort(key=lambda x: x["age"])
print(people)  # [{'name': 'Bob', ...}, {'name': 'Alice', ...}]

Sort complex structures like JSON or database records.

8. Sort Custom Objects with attrgetter

from operator import attrgetter

class Person:
    def __init__(self, name, age):
        self.name = name
        self.age = age

people = [Person("Alice", 30), Person("Bob", 25)]
people.sort(key=attrgetter("age"))

for p in people:
    print(p.name, p.age)

Cleaner and faster than lambda x: x.attr.

9. Stable Sort – Order is Preserved for Equal Items

data = [(2, 'a'), (1, 'b'), (1, 'c')]
sorted_data = sorted(data, key=lambda x: x[0])
print(sorted_data)  # [(1, 'b'), (1, 'c'), (2, 'a')]

Python uses Timsort – a stable sorting algorithm.

Best Practices

Summary – Recap & Next Steps

Python makes sorting intuitive and powerful. You can sort numbers, strings, tuples, dictionaries, and objects with ease using sort() and sorted().

Key Takeaways:

• Use .sort() for in-place sorting
• Use sorted() to get a new list
• Use key functions to customize sorting
• Use reverse=True for descending order
• Python’s sort is stable and fast (Timsort)

Real-World Relevance:
Used in data processing, search filters, leaderboards, sorting records, and ranking systems.

FAQ – Python Sort a List

What is the difference between sort() and sorted()?

• sort() changes the original list
• sorted() returns a new sorted list

How do I sort in descending order?

Use reverse=True:

sorted(data, reverse=True)

Can I sort by multiple keys?

Yes, use a tuple in key:

data.sort(key=lambda x: (x[0], x[1]))

Does sorting change the original list?

• sort() – Yes
• sorted() – No (non-destructive)

Can I sort a list of dictionaries?

Yes, using key=lambda x: x["field"]