Python Classes and Objects

A class 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.

Creating a Class

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

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

Object

An object is a specific instance of a class. It holds its own set of data (instance variables) and can invoke methods defined by its class. Multiple objects can be created from same class, each with its own unique attributes.

Let's create an object from Dog class.

class Dog:
    sound = "bark"

# Creating object from class
dog1 = Dog()  
print(dog1.sound) 

bark

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

Initiate Object with __init__()

__init__() method acts as a constructor and is automatically executed when an object is created. It is used to initialize the attributes of the object with the values provided at the time of object creation.

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)  
print(dog1.species)  

Buddy
Canine

Explanation:

• self refers to the current object and is used to store data inside it.
• Dog("Buddy", 3) creates an object and passes values to __init__().
• self.name and self.age store the object’s name and age.
• dog1.name accesses the object’s instance attribute and dog1.species accesses the shared class attribute.

__str__() Method

__str__() method 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>.

Take an example of using __str__() method to provide a readable string output for an object:

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."
dog1 = Dog("Buddy", 3)
dog2 = Dog("Charlie", 5)

print(dog1)  
print(dog2)  

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

Explanation:

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

To know about Class and Instance variable, refer to this article Class (Static) and Instance Variables in Python.

Difference Between Class and Object

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