Python __lt__() Method
The __lt__() Method
The __lt__() method ("less than") controls what the < operator does for your own objects.
Example
Compare two Person objects by age:
class Person:
def __init__(self, name, age):
self.name = name
self.age = age
def __lt__(self, other):
return self.age < other.age
p1 = Person("Emil", 22)
p2 = Person("Tobias", 19)
print(p1 < p2)
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Compare Without __lt__()
Without __lt__(), comparing two objects with < raises an error:
Example
Remove __lt__(), and the comparison fails:
class Person:
def __init__(self, name, age):
self.name = name
self.age = age
p1 = Person("Emil", 22)
p2 = Person("Tobias", 19)
print(p1 < p2)
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Sorting with __lt__()
Python also uses __lt__() to sort objects, with functions like sorted().
The sorted() function returns a list with the given objects, sorted as specified in the __lt__() method.
Example
Sort a list of Person objects by age:
class Person:
def __init__(self, name, age):
self.name = name
self.age = age
def __lt__(self, other):
return self.age < other.age
p1 = Person("Emil", 22)
p2 = Person("Tobias", 19)
p3 = Person("Linus", 15)
x = sorted([p1, p2, p3])
print(x[0].age)
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Note: sorted() returns a new list, ordered by age.
x[0] is the Person with the lowest age.
You can sort as many objects as you like - not just two or three.
Sorting Without __lt__
The sorted() function relies on <, so without __lt__(), it fails with the same kind of error:
Example
Remove __lt__(), and sorting fails:
class Person:
def __init__(self, name, age):
self.name = name
self.age = age
p1 = Person("Emil", 22)
p2 = Person("Tobias", 19)
p3 = Person("Linus", 15)
x = sorted([p1, p2, p3])
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