Custom Sorting (`key` argument)

Introduction

By default, Python sorts numbers ascending and strings alphabetically.

However, interview questions frequently require complex sorting logic (e.g., sorting intervals by their start time, or strings by their length). Python handles this elegantly via the key argument.

The `key` Argument

Both list.sort() and sorted() accept a key keyword argument.

You provide a function (usually a lambda) to this argument. Python will call this function on every element and sort the elements based on the return value of that function.

Example: Sorting by Length

words = ["banana", "apple", "kiwi", "pear"]

# Sort by the length of the string
words.sort(key=len)
print(words) # ['kiwi', 'pear', 'apple', 'banana']

Sorting Objects and Sub-Arrays

When given an array of arrays (like intervals) or tuples, you often need to sort by a specific index.

Example: Merge Intervals

intervals = [[1, 3], [8, 10], [2, 6], [15, 18]]

# Sort by the start time (index 0)
intervals.sort(key=lambda x: x[0])

print(intervals) # [[1, 3], [2, 6], [8, 10], [15, 18]]

(Note: You can also use operator.itemgetter(0) which is slightly faster than the lambda, but the lambda is universally understood).

Multi-Criteria Sorting

If you need to sort by a primary condition, and then a secondary condition (in case of a tie), your key function should return a tuple.

Python sorts tuples element by element.

Example: Sorting Students

Sort students by Grade (A is better than B), and if grades tie, sort by Age (younger first).

students = [
    {"name": "Alice", "grade": "B", "age": 20},
    {"name": "Bob", "grade": "A", "age": 22},
    {"name": "Charlie", "grade": "A", "age": 19}
]

students.sort(key=lambda s: (s["grade"], s["age"]))

# Charlie comes before Bob because they tie on 'A', but 19 < 22
print(students)

Reversing Specific Criteria

What if you want descending grades, but ascending ages?

For numbers, you simply negate the value (-).

# Sort Age descending, Grade ascending
students.sort(key=lambda s: (s["grade"], -s["age"]))

For strings, negation doesn't work. Instead of complex cmp_to_key workarounds, you can take advantage of Python's stable sorting (explained in the next section) by sorting twice!

Summary

Use key=lambda x: ... to customize sort order.
To sort by multiple criteria, return a tuple: key=lambda x: (x.primary, x.secondary).
Negate numbers -x to reverse the sort order for a specific criteria within a tuple.

Custom Sorting (key argument)