Python Instance Variables

Table of Contents

In the last lesson, we learned about Python Constructors. After learning about constructors, let us learn about instance variables in Python. Instance variables are variables that belong to a specific object.

In this tutorial, we will learn what instance variables are, how to create them, and how they differ from class variables.

What Are Instance Variables in Python?

Instance variables are variables that belong to a specific instance (object) of a class. Each object has its own copy of instance variables.

They are usually created inside the __init__ method using self.

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


dog1 = Dog("Buddy", 3)
dog2 = Dog("Max", 5)


print(dog1.name)   # Buddy
print(dog2.name)   # Max

Output 

Buddy
Max

Here, name and age are instance variables. Each dog has its own name and age.

Why Do We Need Instance Variables in Python?

Instance variables let each object have its own data. Without them, all objects would share the same data. That would be useless.

For example, if all dogs had the same name, we could not tell them apart. Instance variables let us give each dog a different name.

How to Create Instance Variables in Python

We create instance variables by assigning to self inside a method.

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


    def set_age(self, age):
        self.age = age


dog = Dog("Buddy")
dog.set_age(3)


print(dog.name)
print(dog.age)

Output 

Buddy
3

We can also create instance variables outside the class.

class Dog:
    pass


dog = Dog()
dog.name = "Buddy"
dog.age = 3


print(dog.name)
print(dog.age)

Output 

Buddy
3

But it is better to create them in the constructor. That way they are always set.

How to Access Instance Variables in Python

We access instance variables using self inside the class, and using the object name outside the class.

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


    def bark(self):
        print(f"{self.name} says Woof!")


dog = Dog("Buddy")
dog.bark()
print(dog.name)

Output 

Buddy says Woof!
Buddy

Inside the class, we use self.name. Outside, we use dog.name.

How to Modify Instance Variables in Python

We can change instance variables at any time.

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


dog = Dog("Buddy")
print(dog.name)


dog.name = "Max"
print(dog.name)

Output 

Buddy
Max

That is useful when we want to update data.

A Real-Life Example – Student Class with Instance Variables

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


    def introduce(self):
        print(f"Hi, I am {self.name}. I am {self.age} years old. I study {self.course}.")


student1 = Student("Alex", 20, "Computer Science")
student2 = Student("Sam", 22, "Mathematics")


student1.introduce()
student2.introduce()

Output 

Hi, I am Alex. I am 20 years old. I study Computer Science.
Hi, I am Sam. I am 22 years old. I study Mathematics.

Each student has their own instance variables. That is the power of instance variables.

Instance Variables vs Class Variables in Python

Instance variables belong to each object. Class variables belong to the class.

We will learn about class variables in the next tutorial. For now, just remember that instance variables are unique to each object.

Common Mistakes with Instance Variables in Python

Mistake 1: Forgetting self

class Dog:
    def __init__(self, name):
        name = name   # This does not create an instance variable

This creates a local variable, not an instance variable.

Fix:

self.name = name

Mistake 2: Accessing instance variables without an object

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


print(Dog.name)

This gives an error. Because name is an instance variable. It only exists on objects.

Fix:

dog = Dog("Buddy")
print(dog.name)

Mistake 3: Sharing instance variables between objects

class Dog:
    pass


dog1 = Dog()
dog1.name = "Buddy"


dog2 = Dog()
print(dog2.name)

This gives an error. Because dog2 does not have a name attribute. Instance variables are not shared.

Fix: Set the attribute for each object.

dog2.name = "Max"

Next, we will learn about Python Class Variables and share one value across every object of a class.

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