Magic Methods (Dunder Methods) in Python

Magic methods (also called dunder methods) are special methods in Python surrounded by double underscores __.

They allow you to define how objects behave with: - built-in functions - operators - loops - printing - object creation

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Why Magic Methods?

They allow custom classes to behave like built-in types.

Example idea: - + works for numbers → we can define it for objects too - len() works for strings → we can define it for custom objects

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1. __init__() (Constructor)

Used to initialize object values.

Example:

class Student:
    def __init__(self, name, age):
        self.name = name
        self.age = age

s = Student("Aman", 20)
print(s.name, s.age)

Output:

Aman 20

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Real-world example: - Creating user accounts - Initializing product details

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Common Mistake

class A:
    def __init__():
        pass

Error: - missing self

Correct:

def __init__(self):

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Homework:

Create a class Car with brand and price.

Solution:

class Car:
    def __init__(self, brand, price):
        self.brand = brand
        self.price = price

c = Car("Toyota", 500000)
print(c.brand, c.price)

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2. __str__() (User-friendly string)

Used for readable output.

Example:

class Student:
    def __init__(self, name):
        self.name = name

    def __str__(self):
        return f"Student: {self.name}"

s = Student("Aman")
print(s)

Output:

Student: Aman

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Real-world: - Displaying user info in UI - Printing objects cleanly

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3. __repr__() (Developer representation)

Used for debugging and technical representation.

Example:

class Student:
    def __init__(self, name):
        self.name = name

    def __repr__(self):
        return f"Student(name='{self.name}')"

s = Student("Aman")
print(repr(s))

Output:

Student(name='Aman')

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Difference: __str__ vs __repr__

Method	Purpose	Audience
__str__	readable output	user
__repr__	debugging info	developer

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4. __len__() (Length behavior)

Used with len() function.

Example:

class Box:
    def __init__(self, items):
        self.items = items

    def __len__(self):
        return len(self.items)

b = Box([1, 2, 3, 4])
print(len(b))

Output:

4

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Real-world: - Shopping cart items - Inventory count

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5. __iter__() (Iteration support)

Used in loops (for loop support).

Example:

class Numbers:
    def __init__(self):
        self.data = [1, 2, 3]

    def __iter__(self):
        return iter(self.data)

n = Numbers()

for i in n:
    print(i)

Output:

1
2
3

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Real-world: - Database rows - API response iteration

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6. __add__() (Operator overloading)

Defines behavior of + operator.

Example:

class Number:
    def __init__(self, value):
        self.value = value

    def __add__(self, other):
        return self.value + other.value

a = Number(10)
b = Number(20)

print(a + b)

Output:

30

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Real-world: - Adding money objects - Combining vectors - Merging data

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Graph (Operator Flow)

a + b
  ↓
a.__add__(b)
  ↓
custom logic runs

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Combined Example (All Magic Methods)

class Product:
    def __init__(self, name, price):
        self.name = name
        self.price = price

    def __str__(self):
        return f"{self.name} - {self.price}"

    def __repr__(self):
        return f"Product(name='{self.name}', price={self.price})"

    def __len__(self):
        return len(self.name)

    def __add__(self, other):
        return self.price + other.price

p1 = Product("Phone", 10000)
p2 = Product("Laptop", 50000)

print(p1)
print(repr(p1))
print(len(p1))
print(p1 + p2)

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Real World Mapping

Magic Method	Real Use
__init__	object creation
__str__	UI display
__repr__	debugging
__len__	counting items
__iter__	loops
__add__	combining objects

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Common Mistakes

1. Missing self

def __init__():

2. Wrong return type

def __len__(self):
    return "10"

3. Forgetting iterator return

def __iter__(self):
    pass

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Homework 1

Create a class Cart: - store items - support len() - support iteration

Solution:

class Cart:
    def __init__(self, items):
        self.items = items

    def __len__(self):
        return len(self.items)

    def __iter__(self):
        return iter(self.items)

c = Cart(["apple", "banana", "mango"])

print(len(c))

for item in c:
    print(item)

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Homework 2

Create a class Money: - support addition using __add__

Solution:

class Money:
    def __init__(self, amount):
        self.amount = amount

    def __add__(self, other):
        return self.amount + other.amount

a = Money(100)
b = Money(200)

print(a + b)

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Graph (Magic Method System)

Object
  ↓
Magic Method Triggered
  ↓
Python internal behavior overridden

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Next Advanced Topics

• Decorators (deep chaining)
• Context Managers (__enter__, __exit__)
• Iterators & Generators
• Descriptor Protocol
• Metaclasses (very advanced OOP)
• Data classes (@dataclass) ```