ReviseAlgo Logo

Object-Oriented Programming

__init__ Method

Object initialization and constructor patterns

Interview: Critical interview topic — understanding __init__ vs __new__, constructor overloading patterns, and initialization best practices

Last Updated: June 12, 2026 • 7 min read

The __init__ method is Python's constructor — it's automatically called when a new object is created. It initializes the object's attributes and sets up its initial state. Understanding __init__ deeply is crucial for effective OOP in Python.

How __init__ Works

  • __init__ is called after __new__ creates the instance
  • The first parameter is always self (the newly created instance)
  • It should return None — returning anything else raises TypeError
  • It can accept any parameters to customize initialization
  • Default values, args, and *kwargs are all supported

__init__ vs __new__

This is a common interview question. __new__ is responsible for creating the instance, while __init__ is responsible for initializing it. In most cases, you only need __init__. Use __new__ only for immutable types or metaclass programming.

Constructor Patterns

Python doesn't support method overloading, so you can't have multiple __init__ methods. Instead, use default arguments, args/*kwargs, or @classmethod factory methods to provide alternative constructors.

Interview Tip

Be prepared to explain the Singleton pattern using __new__, and know when __init__ gets called multiple times (hint: it can happen with __new__ returning an existing instance).

Use Cases

Initializing object state with required and optional parameters

Implementing Singleton pattern with __new__

Creating factory methods with @classmethod for alternative constructors

Validating input data during object creation

Setting up database connections or file handles in constructors

Common Mistakes

Returning a value from __init__ (other than None) causes TypeError

Using mutable defaults in __init__ parameters (e.g., items=[]) — shared across calls

Forgetting to call super().__init__() in subclass constructors

Confusing __new__ (creates instance) with __init__ (initializes instance)

__init__ is called every time you call the class, even if __new__ returns existing instance