Classes, Instances and the self Parameter

15 min · Beginner

Everything in Python is an object: numbers, strings, lists, functions, and modules. In Object-Oriented Programming (OOP), a class is a blueprint that bundles data (attributes) and behavior (methods) into reusable objects.

Defining a Class and Creating Instances

Class is a blueprint that describes what data and behaviour its objects will have. Writing the class creates nothing yet; calling it makes an object.

Use the class keyword followed by the class name in PascalCase (e.g. BankAccount, UserProfile).

Python

1class Car:2    def drive(self):3        return "The car is driving on the highway."4
5# Create an instance6my_car = Car()7print(my_car.drive())

Example 1 · A class with two methods

Python

1class Invoice:2    def describe(self):3        return "Invoice for a customer order"4
5    def with_tax(self, amount):6        return amount + amount * 18 // 1007
8inv = Invoice()9print(inv.describe())10print(inv.with_tax(1000))

Output

Invoice for a customer order
1180

What this code does

The class lists the behaviour once. Invoice() makes one object that can use it.
self is filled in by Python, so the call passes only 1000.

Car() instantiates a new object in memory. my_car holds a reference to that instance.

Why self is Explicit in Python

self is the first parameter of an instance method. It receives the object the method was called on, so the method can read and change that object.

Unlike languages like Java or C++ where this is implicit and hidden, Python makes self explicit as the first parameter of every instance method.

Python

class Dog:    def bark(self):        print(f"Dog instance at memory address {id(self)} says Woof!")

Example 2 · A method that takes an argument

Python

1class Drone:2    def report_status(self, battery_level):3        return f"Drone active. Battery: {battery_level}%"4
5d = Drone()6print(d.report_status(85))

Output

Drone active. Battery: 85%

What this code does

The Drone class defines the report_status method.
self automatically binds to d when called as d.report_status(85).
The method formats and returns the string.

Example 3 · A method call is a function call in disguise

Python

1class Ticket:2    def label(self):3        return f"Ticket #{self.number}"4
5t = Ticket()6t.number = 77print(t.label())8print(Ticket.label(t))

Output

Ticket #7
Ticket #7

What this code does

t.label() and Ticket.label(t) give the same result.
The first form is what you normally write. Python turns it into the second.

When you call dog.bark(), Python automatically converts it behind the scenes to Dog.bark(dog). self represents the specific instance invoking the method.

Object Identity and the id() Function

Object identity is the single, unchanging id every object has while it exists. Two names with the same identity are two names for one object.

Every object created in Python has a unique integer identity corresponding to its memory location during its lifetime.

Python

car1 = Car()car2 = Car()print(id(car1) != id(car2))  # Trueprint(car1 is car2)          # False

Example 4 · Two objects, one object

Python

1class Customer:2    pass3
4a = Customer()5b = Customer()6c = a7a.name = "Meera"8b.name = "Meera"9print(a is b)10print(a is c)11print(a.name == b.name)

Output

False
True
True

What this code does

a and b are separate objects, so a is b is False even with equal names.
c = a copies the reference, not the object, so a is c is True.

is checks whether two variables point to the exact same object identity in memory.

Dynamic Attribute Assignment

Because Python is dynamic, you can attach attributes directly to an instance at runtime.

Python

car1.color = "Red"print(car1.color)

Example 5 · Attributes belong to one object

Python

1class Account:2    pass3
4x = Account()5y = Account()6x.balance = 5007print(x.balance)8try:9    print(y.balance)10except AttributeError:11    print("y has no balance")

Output

500
y has no balance

What this code does

Setting x.balance adds the attribute to x only.
y was never given one, so reading it raises AttributeError.

While dynamic attributes are possible, defining your attributes systematically inside the constructor (__init__) is the standard practice for clean, predictable architecture.

Next step

Check your understanding

12 quick questions

1class Invoice:2 def describe(self):3 return "Invoice for a customer order"4 5 def with_tax(self, amount):6 return amount + amount * 18 // 1007 8inv = Invoice()9print(inv.describe())10print(inv.with_tax(1000))