Python OOPs Advanced: Class Variables and Instance Variables

We have learned how to create classes and objects. Now, we will explore class variables and instance variables in Python, including how they are used and when to use each one. This chapter also explains how data is stored in variables. If you are still confused about these concepts, this chapter will help clarify them.

What is Variable?

A variable is like a container that stores data or a value. It can store different types of data, such as strings, integers, floats, booleans, and more.

name = "Somit"
age = 30
salary = 50000

Here, name stores a string value, while age and salary store integer values.

Variable in Class and Difference

Inside a class, we use instance variables and class variables. An instance variable belongs to a specific object created from the class. Therefore, when we change an instance variable for one object, it does not affect the same variable in other objects.

While, a class variable is shared by all objects created from the class. It is used when we want to store data that should be common to every object of the class.

Example for Instance Variable

class Hospital:

    def __init__(self, patient, bednumber): 
    	self.patient = patient 
        self.bednumber = bednumber 

WardA = Hospital("Ajay", 32) 
WardB = Hospital("Vijay", 35) 

print(WardA.patient) 
print(WardA.bednumber) 
print(WardB.patient) 
print(WardB.bednumber)

WardA and WardB are objects (instances) of the Hospital class. The __init__ method is the constructor that initializes the instance variables.

In this example, patient and bednumber are instance variables because each object has its own values.

Example for Class Variable

class Hospital:
    Hospital_Name = "Shaurya Hospital"
    def __init__(self, patient, bednumber): 
    	self.patient = patient 
        self.bednumber = bednumber 

WardA = Hospital("Ajay", 32) 
WardB = Hospital("Vijay", 35) 

print(WardA.Hospital_Name)  
print(WardB.Hospital_Name)

Change Class Variable

Hospital.Hospital_Name ="Agam Hospital"
##Check object
print(WardA.Hospital_Name)
print(WardB.Hospital_Name)

Changing a class variable affects all objects (instances) of that class that use the variable.

Quick Difference

Instance Variable	Class Variable
Belongs to an object	Belongs to the class
Each object has its own copy	One copy shared by all objects
Created using self.variable	Created directly inside the class
Changing one object doesn't affect others	Changing it affects all objects

More Examples

class Restaurant:
    Name_Restaurant = "Annapurna Restaurant"

    def __init__(self, snacks, price):
        self.snacks = snacks
        self.price = price

Snack1 = Restaurant("Burger", 50)
Snack2 = Restaurant("Dosa", 100)
Snack3 = Restaurant("Idli", 30)

print(Snack3.Name_Restaurant)
print(Snack1.snacks)
print(Snack1.price)

Note: If the constructor has additional parameters, you must provide values for them when creating the object, unless those parameters have default values.

For example: def __init__(self, snacks, price, rating=0):

class Admissions:
    College = "Chhatrapati Shahu Ji Maharaj University"
    Total_count = 0

    def __init__(self, name, course):
        self.name = name
        self.course = course
        Admissions.Total_count += 1

Science1 = Admissions("Amit", "BSc.")
Science2 = Admissions("Rohit", "BSc.")
Science3 = Admissions("Priya", "Bsc.")

print(Admissions.Total_count)

Note: If we use the same object variable name again, the previous reference is replaced by the new object.

Conclusion

Continue engage your knowledge in python and practice as much as you can. The next chapter will be: Python OOPs Advanced: Class Methods (@classmethod)

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