How to Make Custom Class Sortable in Python: Simple Guide

To make a custom class sortable in Python, define the __lt__ method to specify how objects compare using the less-than operator. Optionally, implement other comparison methods like __eq__ for full sorting support. Then, you can use built-in functions like sorted() on your class instances.

Syntax

Define the __lt__(self, other) method in your class to tell Python how to compare two objects using the less-than operator (<). This method should return True if self is considered smaller than other, otherwise False.

Optionally, define __eq__(self, other) to specify when two objects are equal. This helps sorting functions handle ties properly.

python

class MyClass:
    def __init__(self, value):
        self.value = value

    def __lt__(self, other):
        return self.value < other.value

    def __eq__(self, other):
        return self.value == other.value

Example

This example shows a custom class Person with an age attribute. We define __lt__ and __eq__ so that sorting a list of Person objects sorts them by age.

python

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

    def __lt__(self, other):
        return self.age < other.age

    def __eq__(self, other):
        return self.age == other.age

    def __repr__(self):
        return f"Person({self.name}, {self.age})"

people = [Person("Alice", 30), Person("Bob", 25), Person("Charlie", 35)]
sorted_people = sorted(people)
print(sorted_people)

Output

[Person(Bob, 25), Person(Alice, 30), Person(Charlie, 35)]

Common Pitfalls

• Not defining __lt__ causes TypeError when sorting your objects.
• Defining only __lt__ without __eq__ can cause unexpected behavior in some sorting or comparison operations.
• Comparing incompatible types inside __lt__ can raise errors; always check the type of other if needed.

python

class Wrong:
    def __init__(self, value):
        self.value = value

# Missing __lt__ method

items = [Wrong(1), Wrong(2)]

# This will raise TypeError:
# sorted(items)

# Correct way:
class Right:
    def __init__(self, value):
        self.value = value
    def __lt__(self, other):
        return self.value < other.value

items = [Right(1), Right(2)]
print(sorted(items))

Output

[<__main__.Right object at 0x...>, <__main__.Right object at 0x...>]

Quick Reference

To make a class sortable:

• Define __lt__(self, other) to compare objects.
• Optionally define __eq__(self, other) for equality checks.
• Use sorted() or .sort() on lists of your objects.

Remember: __lt__ means "less than" and controls sorting order.

Key Takeaways

Define __lt__ method in your class to enable sorting by specifying less-than comparison.

Implement __eq__ method to support equality checks and improve sorting behavior.

Use built-in sorted() or list.sort() to sort your custom objects once comparison methods are defined.

Avoid TypeError by ensuring comparison methods handle only compatible types.

Without __lt__, Python cannot sort your custom objects and will raise errors.