FutureMind Academy

Last Updated on August 1, 2025 by Rajeev Bagra

When working with Python, especially as a beginner, you might wonder:

What’s the difference between a regular add() function and the special __add__() method?

They both seem to involve addition — but they serve different purposes. Let’s explore how they work, when to use each, and why dunder methods (those double underscore methods like __add__) are vital in object-oriented programming.

What Is a Regular add() Function?

A simple Python function that adds two values might look like this:

def add(a, b):     return a + b  print(add(3, 4))  # Output: 7 

This works perfectly for built-in data types like integers, floats, strings, and lists — because Python already knows how to add them.

What Happens Behind the Scenes?

When Python sees a + b, it internally translates that to:

a.__add__(b) 

So even though you’re writing a + b, Python is actually calling the special dunder method __add__.

a = 3 b = 4  print(a + b)           # Output: 7 print(a.__add__(b))    # Also 7 — same thing! 

The Limitation of add() — Custom Classes

Now imagine you define a custom class, like a 2D point:

class Point:     def __init__(self, x, y):         self.x = x         self.y = y 

Trying to do this:

p1 = Point(1, 2) p2 = Point(3, 4) print(p1 + p2)  #  ERROR 

Python throws an error because it doesn’t know how to add two Point objects.

Overcoming the Limitation with __add__()

You can tell Python how to handle + by defining the __add__ dunder method inside your class:

class Point:     def __init__(self, x, y):         self.x = x         self.y = y      def __add__(self, other):         return Point(self.x + other.x, self.y + other.y)      def __str__(self):         return f"Point({self.x}, {self.y})" 

Now this works:

p1 = Point(1, 2) p2 = Point(3, 4) p3 = p1 + p2 print(p3)  # Output: Point(4, 6) 

Magic! The + operator is now meaningful for your custom object.

Why Use __add__() When add() Exists?

class Product:     def __init__(self, name, price):         self.name = name         self.price = price      def __add__(self, other):         return self.price + other.price      def __mul__(self, quantity):         return self.price * quantity      def __eq__(self, other):         return self.name == other.name and self.price == other.price      def __lt__(self, other):         return self.price < other.price      def __str__(self):         return f"{self.name} (${self.price})" 

p1 = Product("Book", 10) p2 = Product("Pen", 2) print(p1 + p2)  # 12 print(p2 * 5)   # 10 print(p1 == p2) # False