Python __lt__() Magic Method

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Short summary:

  • To customize the behavior of the less than operator x < y, override the __lt__() dunder method in your class definition.
  • Python internally calls x.__lt__(y) to obtain a return value when comparing two objects using x < y.
  • The return value can be any data type because any value can automatically converted to a Boolean by using the bool() built-in function.
  • If the __lt__() method is not defined, Python will raise a TypeError.

Syntax

__lt__(self, other)

To use the less than operator on custom objects, define the __lt__() “dunder” magic method that takes two arguments: self and other. You can then use attributes of the custom objects to determine if one is less than the other.

The method should return a Boolean True or False — however, this is not required because every object can be automatically converted to a Boolean value using the built-in bool() function.

Let’s have a look at an example next.

Example

In the following code, you compare two persons with each other by using the age attribute as a decision criterion:

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

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



alice = Person(18)
bob = Person(17)
carl = Person(18)

print(alice < bob)
# False

print(alice < carl)
# False

print(bob < alice)
# True

For example, because Alice’s age is 18 years and Bob’s 17 years, the expression alice < bob evaluates to True.

Background Video

Python Less Than - Comparison Operator

Default Implementation of __lt__

The __lt__() dunder method doesn’t have a default implementation. If you try to compare objects using the less than operator <, Python will simply raise a TypeError. 

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


alice = Person(18)
bob = Person(17)
carl = Person(18)

print(alice < bob)
Traceback (most recent call last):
  File "C:\Users\...\code.py", line 10, in <module>
    print(alice < bob)
TypeError: '<' not supported between instances of 'Person' and 'Person'

TypeError: ‘<‘ not supported between instances of ‘…’ and ‘…’

If you get the TypeError: '<' not supported between instances of '...' and '...', you try to compare two objects using the less than operator x < y for which the __lt__() magic method is not defined.

class Finxter:
    pass


x = Finxter()
y = Finxter()

x < y    # Python will raise an error!

Output:

Traceback (most recent call last):
  File "C:\Users\...\code.py", line 8, in <module>
    x < y    # Python will raise an error!
TypeError: '<' not supported between instances of 'Finxter' and 'Finxter'

To fix the error, simply define the __lt__(self, other) method in your class definition and return any object that will then be converted to a Boolean True or False. 

class Finxter:
    def __lt__(self, other):
        return 42


x = Finxter()
y = Finxter()

x < y    # Now it works!
# 42

Commutativity of Less Than <

The output of x < y and y < x may be different because the former calls x.__lt__(y) and the latter calls y.__lt__(x). If x and y have different definitions of the dunder method __lt__(), the operation becomes non-commutative.

You can see this in the following example:

class Person:
    def __lt__(self, other):
        return False


class Human:
    def __lt__(self, other):
        return True


alice = Person()
bob = Human()


print(alice < bob)
# False

print(bob < alice)
# True

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