Python Classes and Objects

Last Updated : 10 Mar, 2025

A class in Python is a user-defined template for creating objects. It bundles data and functions together, making it easier to manage and use them. When we create a new class, we define a new type of object. We can then create multiple instances of this object type.

Classes are created using class keyword. Attributes are variables defined inside the class and represent the properties of the class. Attributes can be accessed using the dot . operator (e.g., MyClass.my_attribute).

Create a Class

# define a class
class Dog:
    sound = "bark"  # class attribute

Create Object

An Object is an instance of a Class. It represents a specific implementation of the class and holds its own data.

Now, let’s create an object from Dog class.

class Dog:
    sound = "bark" 

# Create an object from the class
dog1 = Dog()

# Access the class attribute
print(dog1.sound)

sound attribute is a class attribute. It is shared across all instances of Dog class, so can be directly accessed through instance dog1.

Using init() Function

In Python, class has __init__() function. It automatically initializes object attributes when an object is created.

class Dog:
    species = "Canine"  # Class attribute

    def __init__(self, name, age):
        self.name = name  # Instance attribute
        self.age = age  # Instance attribute

Explanation:

class Dog: Defines a class named Dog.
species: A class attribute shared by all instances of the class.
__init__ method: Initializes the name and age attributes when a new object is created.

Initiate Object with init

class Dog:
    species = "Canine"  # Class attribute

    def __init__(self, name, age):
        self.name = name  # Instance attribute
        self.age = age  # Instance attribute

# Creating an object of the Dog class
dog1 = Dog("Buddy", 3)

print(dog1.name)  # Output: Buddy
print(dog1.species)  # Output: Canine

Output

Buddy
Canine

Explanation:

dog1 = Dog(“Buddy”, 3): Creates an object of the Dog class with name as “Buddy” and age as 3.
dog1.name: Accesses the instance attribute name of the dog1 object.
dog1.species: Accesses the class attribute species of the dog1 object.

Self Parameter

self parameter is a reference to the current instance of the class. It allows us to access the attributes and methods of the object.

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

    def bark(self): 
        print(f"{self.name} is barking!")

# Creating an instance of Dog
dog1 = Dog("Buddy", 3)
dog1.bark()

Output

Buddy is barking!

Explanation:

Inside bark(), self.name accesses the specific dog’s name and prints it.
When we call dog1.bark(), Python automatically passes dog1 as self, allowing access to its attributes.

str Method

__str__ method in Python allows us to define a custom string representation of an object. By default, when we print an object or convert it to a string using str(), Python uses the default implementation, which returns a string like <__main__.ClassName object at 0x00000123>.

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

    def __str__(self):
        return f"{self.name} is {self.age} years old."  # Correct: Returning a string
      
dog1 = Dog("Buddy", 3)
dog2 = Dog("Charlie", 5)

print(dog1)  
print(dog2)

Output

Buddy is 3 years old.
Charlie is 5 years old.

Explanation:

__str__ Implementation: Defined as a method in the Dog class. Uses the self parameter to access the instance’s attributes (name and age).
Readable Output: When print(dog1) is called, Python automatically uses the __str__ method to get a string representation of the object. Without __str__, calling print(dog1) would produce something like <__main__.Dog object at 0x00000123>.

Class and Instance Variables in Python

In Python, variables defined in a class can be either class variables or instance variables, and understanding the distinction between them is crucial for object-oriented programming.

Class Variables

These are the variables that are shared across all instances of a class. It is defined at the class level, outside any methods. All objects of the class share the same value for a class variable unless explicitly overridden in an object.

Instance Variables

Variables that are unique to each instance (object) of a class. These are defined within __init__ method or other instance methods. Each object maintains its own copy of instance variables, independent of other objects.

Example:

class Dog:
    # Class variable
    species = "Canine"

    def __init__(self, name, age):
        # Instance variables
        self.name = name
        self.age = age

# Create objects
dog1 = Dog("Buddy", 3)
dog2 = Dog("Charlie", 5)

# Access class and instance variables
print(dog1.species)  # (Class variable)
print(dog1.name)     # (Instance variable)
print(dog2.name)     # (Instance variable)

# Modify instance variables
dog1.name = "Max"
print(dog1.name)     # (Updated instance variable)

# Modify class variable
Dog.species = "Feline"
print(dog1.species)  # (Updated class variable)
print(dog2.species)

Output

Canine
Buddy
Charlie
Max
Feline
Feline

Explanation:

Class Variable (species): Shared by all instances of the class. Changing Dog.species affects all objects, as it’s a property of the class itself.
Instance Variables (name, age): Defined in the __init__ method. Unique to each instance (e.g., dog1.name and dog2.name are different).
Accessing Variables: Class variables can be accessed via the class name (Dog.species) or an object (dog1.species). Instance variables are accessed via the object (dog1.name).
Updating Variables: Changing Dog.species affects all instances. Changing dog1.name only affects dog1 and does not impact dog2.