Magic (Dunder) Methods in Python

What are magic methods

Magic methods (also called dunder methods, for "double underscore") are special methods with names like __str__ and __add__. You never call them directly — Python calls them automatically when you print an object, add two objects, check their length, and more. They let your class behave like a built-in type.

__str__ — printable text

class Book:
    def __init__(self, title):
        self.title = title
    def __str__(self):
        return "Book: " + self.title

b = Book("Python 101")
print(b)

__str__ decides what print(b) shows. Without it, printing an object gives an unhelpful memory address; with it, you get clean, readable output.

__len__ — length

class Team:
    def __init__(self, members):
        self.members = members
    def __len__(self):
        return len(self.members)

t = Team(["Aman", "Diya", "Kabir"])
print(len(t))

__len__ is called when you use len(t) on your object. Here it returns how many members the team has, so len() works naturally.

__add__ — the + operator

class Money:
    def __init__(self, amount):
        self.amount = amount
    def __add__(self, other):
        return Money(self.amount + other.amount)

total = Money(100) + Money(50)
print(total.amount)

__add__ defines what + means for your class — this is called operator overloading. Now Money(100) + Money(50) makes sense and returns a new Money of 150.

__eq__ — comparison

class Point:
    def __init__(self, x, y):
        self.x, self.y = x, y
    def __eq__(self, other):
        return self.x == other.x and self.y == other.y

print(Point(1, 2) == Point(1, 2))

__eq__ defines what == means. By default two objects are equal only if they are the exact same object; here we compare by value, so two points with the same coordinates are equal.

Common mistakes

• Calling dunder methods directly. Write print(b), not b.__str__() — Python calls them for you.
• __str__ not returning a string. It must return text, not print it.
• Wrong spelling. Two underscores each side: __add__, not _add_.
• Forgetting other. Methods like __add__ and __eq__ take a second object to work with.

Summary

• Magic (dunder) methods have double underscores and are called automatically.
• __str__ controls what print() shows.
• __len__ makes len() work on your object.
• __add__ defines the + operator (operator overloading).
• __eq__ defines how == compares your objects.

Next, revise classes with self and __init__, or extend them with inheritance and polymorphism.