self and __init__ in Python

What are self and __init__

When you build a class in Python, two things appear again and again: __init__ and self. __init__ is the constructor — it sets up a new object. self is how a method refers to the object it belongs to. Together they let each object hold its own data.

The __init__ constructor

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

s = Student("Aman", 12)
print(s.name, s.roll)

__init__ runs automatically when you write Student("Aman", 12). It receives the values and stores them on the object with self.name and self.roll.

Why every method needs self

class Student:
    def __init__(self, name):
        self.name = name
    def greet(self):
        return "Hi, " + self.name

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

Every method takes self first, so it knows which object it is working on. self.name reads that object's own name. When you call s.greet(), Python passes s as self automatically.

Each object has its own data

a = Student("Aman")
b = Student("Diya")
print(a.greet())    # Hi, Aman
print(b.greet())    # Hi, Diya

Because data is stored on self, each object keeps its own copy. a and b are separate students with separate names — that is the whole point of self.

Default values in __init__

class Student:
    def __init__(self, name, marks=0):
        self.name = name
        self.marks = marks

s = Student("Aman")       # marks defaults to 0
print(s.name, s.marks)

You can give a parameter a default in __init__, so it is optional. Here marks defaults to 0 if the caller does not provide it.

Common mistakes

• Forgetting self in a method. Every method needs it as the first parameter.
• Passing self in the call. Write obj.greet(), not obj.greet(obj) — Python adds it.
• Using name instead of self.name. Without self, it is a temporary local variable.
• Wrong dunder spelling. It is __init__ with two underscores each side.

Summary

• __init__ is the constructor — it runs when an object is created.
• self means "this object" and is every method's first parameter.
• Store data with self.attribute so each object keeps its own.
• Python passes self automatically when you call a method.
• You can give __init__ parameters default values.

Next, extend classes with inheritance and polymorphism, or explore magic methods.