Magic (Dunder) Methods in Python

In Python’s object-oriented model, objects can interact with built-in language features such as print(), len(), comparison operators, and more. This interaction is enabled through special methods, commonly called Magic Methods or Dunder Methods (Double-UNDERscore methods).

These methods allow developers to define how objects behave when used with Python’s built-in operations.

For example:

• print(obj) → calls __str__()
• len(obj) → calls __len__()
• obj1 == obj2 → calls __eq__()

Magic methods make custom objects behave like built-in Python types such as lists, strings, or dictionaries.

Understanding Magic (Dunder) Methods

Magic methods are predefined method names in Python that begin and end with double underscores.

Example:

__init__
__str__
__repr__
__len__
__eq__

These methods are not meant to be called directly. Instead, Python automatically invokes them when certain operations occur.

Example:

class Demo:
  def __str__(self):
    return "Demo Object"

d = Demo()
print(d)# internally becomes d.__str__()

Why Magic Methods Are Important

Magic methods allow developers to:

• Customize object behavior
• Integrate custom classes with Python’s built-in functions
• Implement operator overloading
• Build intuitive APIs
• Make objects behave like built-in data types

Without magic methods, custom classes would be much less flexible.

1. __str__() — Human-Readable Representation

__str__() returns a human-friendly string representation of an object.

This method is automatically called by:

• print(obj)
• str(obj)
• format(obj)

The goal of __str__() is to provide a clean and readable description of the object.

Syntax:

def __str__(self):
    return "string representation"

Example:

class Student:

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

    def __str__(self):
        return f"Student(Name={self.name}, Age={self.age})"


s = Student("Alice", 22)
print(s)
# output:
  # Student(Name=Alice, Age=22)

Without __str__()

If __str__() is not implemented:

print(s)
# Output:
  # <__main__.Student object at 0x0000018F2D3>

This output is not useful for users.

2. __repr__() — Developer-Focused Representation

__repr__() returns the official string representation of an object intended for developers and debugging.

It is used by:

• Python interpreter
• Debugging tools
• repr(obj)
• Interactive shell

The representation should ideally allow recreating the object.

Get Saurabh Gandhi’s stories in your inbox

Join Medium for free to get updates from this writer.

Remember me for faster sign in

Syntax:

def __repr__(self):
    return "representation"

Example:

class Student:

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

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

s = Student("Alice", 22)
repr(s)
# output:
  # Student('Alice', 22)

__repr__() vs __str__()

Example:

class Product:

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

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

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


p = Product("Laptop", 75000)

print(p)
repr(p)

3. __len__() — Defining Object Length

__len__() allows an object to define its length.

It is invoked when calling:

len(object)

This method must return a non-negative integer.

Example:

class Library:

    def __init__(self, books):
        self.books = books

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


lib = Library(["Python", "ML", "AI"])

print(len(lib))

Practical Example — Shopping Cart

class ShoppingCart:

    def __init__(self):
        self.items = []

    def add_item(self, item):
        self.items.append(item)

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


cart = ShoppingCart()

cart.add_item("Laptop")
cart.add_item("Mouse")

print(len(cart))

4. __del__() — Object Destructor

__del__() is a destructor method called when an object is about to be destroyed.

It is used for:

• releasing resources
• closing files
• cleaning up connections

Syntax:

def __del__(self):
    # cleanup code

Example:

class FileHandler:

    def __init__(self, filename):
        self.file = open(filename, "w")

    def __del__(self):
        print("Closing file")
        self.file.close()


f = FileHandler("test.txt")
del f

Important Concept: Garbage Collection

Python uses reference counting and garbage collection to destroy objects.

Therefore, __del__() execution time is not always predictable.

This means:

__del__() is rarely used in production systems

Instead developers use context managers (with statement).

5. __eq__() — Custom Equality Comparison

__eq__() defines how two objects should be compared using ==.

Syntax:

def __eq__(self, other):

Example:

class Employee:

    def __init__(self, emp_id):
        self.emp_id = emp_id

    def __eq__(self, other):
        return self.emp_id == other.emp_id


e1 = Employee(101)
e2 = Employee(101)

print(e1 == e2)

Default Behavior Without __eq__

Without defining __eq__, Python compares memory addresses.

Example:

class Test:
    pass

a = Test()
b = Test()

print(a == b)

Execution Flow of Magic Methods

User Operation
      |
      v
Python Built-in Function
      |
      v
Magic Method Triggered

Example:

print(obj)      → obj.__str__()
repr(obj)       → obj.__repr__()
len(obj)        → obj.__len__()
del obj         → obj.__del__()
obj1 == obj2    → obj.__eq__()

Questions

1. What are magic methods in Python?
2. Why are they called “dunder methods”?
3. What is the purpose of __str__()?
4. Difference between __str__() and __repr__()?
5. When does Python call __len__()?
6. What happens if __eq__() is not implemented?

Table of Contents

• Object-Oriented Programming (OOP) in Python
• Python: A Beginner’s Guide to Programming