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Python Program to Show Operator Overloading

Intermediate · Classes and objects

Operator overloading means giving operators such as +, - and == a meaning for objects of your own class. Python does this through special methods, also called magic or dunder methods, whose names start and end with two underscores. This program makes a Vector class whose objects can be added, subtracted, multiplied, compared and printed.

Program

operator_overloading.py
class Vector:
def __init__(self, x, y):
self.x = x
self.y = y
def __add__(self, other): # v1 + v2
return Vector(self.x + other.x, self.y + other.y)
def __sub__(self, other): # v1 - v2
return Vector(self.x - other.x, self.y - other.y)
def __mul__(self, number): # v * 3
return Vector(self.x * number, self.y * number)
def __eq__(self, other): # v1 == v2
return self.x == other.x and self.y == other.y
def __str__(self): # print(v)
return f"Vector({self.x}, {self.y})"
a = Vector(2, 3)
b = Vector(5, 1)
print("a =", a)
print("b =", b)
print("a + b =", a + b)
print("a - b =", a - b)
print("a * 3 =", a * 3)
print("a == Vector(2, 3):", a == Vector(2, 3))
Output
a = Vector(2, 3)
b = Vector(5, 1)
a + b = Vector(7, 4)
a - b = Vector(-3, 2)
a * 3 = Vector(6, 9)
a == Vector(2, 3): True

How it works

  1. When Python sees a + b with two objects, it calls a.__add__(b), so defining __add__ decides what + does for vectors.
  2. __add__, __sub__ and __mul__ each return a new Vector, so a and b stay unchanged.
  3. __eq__ makes == compare the coordinates; without it, two separate objects with the same values would not count as equal.
  4. __str__ sets what print() shows, here Vector(7, 4) instead of an unreadable memory address.
  5. a * 3 works, but 3 * a raises an error unless the class also defines __rmul__, the method for a number on the left.

Other ways to write it

Overloading < to sort objects

Overloading < to sort objects
class Student:
def __init__(self, name, marks):
self.name = name
self.marks = marks
def __lt__(self, other): # s1 < s2 compares marks
return self.marks < other.marks
def __repr__(self):
return f"{self.name}({self.marks})"
students = [Student("Aarav", 82), Student("Diya", 95), Student("Kabir", 71)]
print("Sorted by marks:", sorted(students))
print("Highest:", max(students))
print("Is Diya below Aarav?", students[1] < students[0])
Output
Sorted by marks: [Kabir(71), Aarav(82), Diya(95)]
Highest: Diya(95)
Is Diya below Aarav? False

Defining __lt__ (less than) is enough for sorted(), min() and max() to work on your objects, and __repr__ is what Python shows for objects inside a list.

Adding durations

Adding durations
class Duration:
def __init__(self, hours, minutes):
self.hours = hours + minutes // 60 # carry whole hours over
self.minutes = minutes % 60
def __add__(self, other):
return Duration(self.hours + other.hours, self.minutes + other.minutes)
def __str__(self):
return f"{self.hours} h {self.minutes} min"
movie = Duration(2, 35)
ads = Duration(0, 40)
print("Movie:", movie)
print("Ads:", ads)
print("Total:", movie + ads)
Output
Movie: 2 h 35 min
Ads: 0 h 40 min
Total: 3 h 15 min

__add__ adds the hours and minutes, and the constructor carries every 60 minutes over into an hour, so 2 h 35 min + 40 min gives 3 h 15 min.

Try it yourself

Add a __neg__ method so that -a gives Vector(-2, -3).

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