How to Sort Using Lambda in Python: Simple Guide

In Python, you can sort a list using the sorted() function or the .sort() method with a key argument set to a lambda function. The lambda defines the sorting criteria by returning the value to sort by for each element.

Syntax

The basic syntax to sort a list using a lambda function is:

• sorted(iterable, key=lambda x: expression) - returns a new sorted list.
• list.sort(key=lambda x: expression) - sorts the list in place.

The lambda x: expression is an anonymous function that takes each element x and returns the value used for sorting.

python

sorted_list = sorted(my_list, key=lambda x: x[1])
my_list.sort(key=lambda x: x[1])

Example

This example shows how to sort a list of tuples by the second item using a lambda function.

python

data = [(1, 'banana'), (3, 'apple'), (2, 'cherry')]
sorted_data = sorted(data, key=lambda x: x[1])
print(sorted_data)

Output

[(3, 'apple'), (1, 'banana'), (2, 'cherry')]

Common Pitfalls

Common mistakes include:

• Not using the key argument and expecting lambda to sort directly.
• Modifying the list inside the lambda, which should only return a value.
• Confusing sorted() (returns new list) with .sort() (modifies in place).

python

data = [3, 1, 2]
# Wrong: lambda alone does not sort
# sorted(data)  # sorts by value, no lambda used

# Right: use lambda with key
sorted_data = sorted(data, key=lambda x: -x)  # sorts descending
print(sorted_data)

Output

[3, 2, 1]

Quick Reference

Summary tips for sorting with lambda:

• Use key=lambda x: ... to specify sorting criteria.
• sorted() returns a new sorted list.
• .sort() changes the original list.
• Lambda should return the value to sort by, not modify data.

Key Takeaways

Use the key argument with a lambda function to define custom sorting criteria.

sorted() returns a new list, while list.sort() modifies the list in place.

Lambda functions should only return the sorting key, not change the data.

Remember to use key=lambda x: expression, not just lambda alone.

Sorting by complex criteria is easy by adjusting the lambda expression.