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home / developersection / interviews / what is python __init__()?
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Jk Malhotra
Jk Malhotra

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1 Answers

Jk Malhotra
Jk Malhotra 23 Oct 2025
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The __init__() method is a special method (also called a “dunder” method) in Python classes.
It automatically runs every time you create a new object from a class.

  • It is known as the constructor (short for “initializer”).
  • It’s used to initialize object attributes — i.e., set up the object with specific data when it’s created.

Basic Example

class Person:
    def __init__(self, name, age):  # constructor
        self.name = name            # instance attribute
        self.age = age

# create object (this calls __init__ automatically)
p1 = Person("Alice", 25)
p2 = Person("Bob", 30)

print(p1.name)   # Alice
print(p2.age)    # 30

When you call Person("Alice", 25), Python does this behind the scenes:

  • Creates a new empty Person object in memory.
  • Calls __init__(p1, "Alice", 25) to initialize it.
  • Returns the fully constructed object.

What is self?

The self parameter refers to the current instance (object) being created. You must include it as the first parameter in any instance method, including __init__(). It lets you store data on that specific object.

Example:

class Dog:
    def __init__(self, name):
        self.name = name   # binds the value to the object

dog1 = Dog("Buddy")
dog2 = Dog("Max")

print(dog1.name)   # Buddy
print(dog2.name)   # Max

Each dog keeps its own name value because of self.

Default and Optional Parameters

You can give parameters default values, just like with normal functions.

class Car:
    def __init__(self, brand, color="white"):
        self.brand = brand
        self.color = color

c1 = Car("Toyota", "red")
c2 = Car("Honda")  # uses default color

print(c1.color)  # red
print(c2.color)  # white

__init__() vs Normal Methods

Feature __init__() Normal Method
Called automatically? Yes (on object creation) No (you call it manually)
Purpose Initialize object attributes Perform actions on an object
Returns Always None Can return values
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