Summary: In this tutorial, we will learn what class and instance variables are in Python and how they differ from each other in context to programming.

Python being an object-oriented programming language allows variables to be used at class level or at the instance level.

When the variables are declared at the class level they are referred to as class variables, whereas when they are declared at instance level they are referred to as instance variables.

We declare variables at class level, when we expect all the objects (instances of a class) to share a single copy of a variable, whereas we declare variables at instance level when each object needs to have a separate copy of the variable, so that they can have different values stored in them.

Let’s understand with examples.

Recommended: object oriented programming in Python

Class Variable

Class variables are shared by all the instances of the class (objects) because they are owned by the class. If it gets changed by any single object it changes for all objects.

Class variables are declared within class outside of any methods, usually just below the class header.

class Book:
    book_type = "programming"

Here book_type is a class variable assigned with “programming”.

Let’s create some instances of the class and output their book_type value.

class Book:
    book_type = "programming"

python_book = Book()
java_book = Book()
c_book = Book()




We can clearly see that all the instances have the same value because book_type is a class variable.

Just like book_type, we can declare multiple class variables inside of the class and each of them will be accessible (and shared) to each instance of the class.

Furthermore, since class variables are not bound to any instance, they can also be accessed via class name or reference.

class Book:
    book_type = "Sci-Fi"
print(Book.book_type)   #outputs Sci-Fi

We can define variables in the class’s scope by taking the advantage of the class reference (i.e. cls or self.__class__) in Python.

In this example, we have defined a class variable using self.__class__ and later modified its value via cls within the class method:

class Student:
    def __init__(self):
        #self.__class__ is the class reference = "Pencil Programmer School"
    def changeSchoolNameTo(cls, name): = name
stud1 = Student()
stud2 = Student()

#changing the class variable school value via stud1 object
stud1.changeSchoolNameTo("No Fee Programming School")

#since class variable is shared among all the objects,
#we can notice the change made by stud1 in the stud2 object.
print(         #outputs "No Fee Programming School"

Instance Variable

Instance variables are the properties of objects in Python. This means that every object or instance of the class maintains a separate copy of the instance variable.

Instance variables are declared within the instance method using the self argument.

class Book:
    def __init__(self, bt):
        self.book_type = bt

Here book_type is an example of instance variable.

We can define instance variable in any instance method in Python. It is not mandatory to initialize the instance variable only in the __init__ function.

class Book:
    def __init__(self, bt):
        self.book_type = bt
    def setTitle(self, title):
        self.title = title

book = Book("Programming")
book.setTitle("Go Hero in Python")

print(book.title)         #outputs "Go Hero in Python"
print(book.book_type)     #outputs "Programming"

In this example, we have defined two instance variables in two different instance methods.

Now, let’s create some more instances of the class.

class Book:
    def __init__(self, bt):
        self.book_type = bt

python = Book("Programming")
recepie = Book("Cooking")
Maharanapratap = Book("Biography")




You can see that each instance of the same class has different values of book_type property, which was passed during the creation of instances.

Hence, it is proved that the instance variables are owned by the objects.

Using Class and Instance variable Together

Now as we have some idea of class and instance variable, let’s see how they can be used together.

class Book:
    publication = "Pencil_Programmer_Publication"
    def __init__(self, bt):
        self.book_type = bt

python = Book("programming")
Maharanapratap = Book("Biography")

print("Python: "+python.book_type+" - "+python.publication)
print("Maharanapratap: "+ Maharanapratap.book_type +" - "+ Maharanapratap.publication)


Python: programming – Pencil_Programmer_Publication
Maharanapratap: Biography – Pencil_Programmer_Publication

In the above example, we have created two Books of the same publication but of different genre.

Since their publication are same but the type is different, we have defined publication as the class variable and book_type as the instance variable.

This means that the value of publication will be shared (same for both instances) and the book_type will be different for both.

When to use class or instance variable?

We should use class variable when we want to have same value for all the instances (objects), otherwise use an instance variable.

This Post Has 2 Comments

  1. Pinkesh Shah

    Great article on variables of Python !

    I have a question on class variables not being accessible to class methods. We can use instance variables within the method but not class variables.

    I will appreciate to know about its functionality in Python programming.

    Thank You

    1. Adarsh Kumar

      I think you are confused with class methods.

      I recommend you check out what actually are the class methods in Python?

      class methods are meant to access the class variables but not the instance variable.

      Normal function inside class has access to self parameter which can be used to access class as well as the instance variables but in class methods cls parameter is passed instead of self which has access to class variables only.

      Remember, class variable is accessible to any function unless it is not static and instance variable is accessible to only instance methods (normal methods inside a class with self parameter).

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