Class Variables in Python: Everything You Need to Know
Python is a popular programming language known for its versatility and ease of use. It is widely used for developing web applications, scientific computing, artificial intelligence, and data science.
This article will delve into one of the essential concepts of Python programming – Class Variables. This article aims to provide a comprehensive understanding of what class variables are, how they differ from instance variables, how to create them, access them, and how they behave in inheritance.
Class Variables
Class Variables, also known as Static Variables, are an essential part of Python programming. They hold values that are shared by all instances of a class.
Class Variables belong to the class and not a specific instance of the class. Their primary purpose is to maintain the state of the class and its behavior.
Instance Variables vs Class Variables
Before we dive deeper into Class Variables, it is essential to understand the difference between Instance Variables and Class Variables. Instance variables hold data that is unique to each instance of a class.
This means that every object or instance of a class can have different values for instance variables. On the other hand, Class Variables are shared by all instances of a class.
This means that every object of a class will have the same value for Class Variables.
Behaviour of Class Variables in Inheritance
Inheritance is one of the fundamental concepts of Object-Oriented Programming (OOP). Inheritance allows a class to inherit properties and methods from another class.
When a subclass inherits from a superclass, it also inherits its Class Variables. All instances of the subclass share the same Class Variable as the superclass.
However, if the subclass declares a new Class Variable with the same name as the superclass, the subclass’s Class Variable overrides the superclass’s Class Variable.
Creating Class Variables
Creating Class Variables is easy. You can add them to the class header, and they will be available to all instances of the class.
You can initialize Class Variables in the __init__() method or anywhere in the class.
Accessing Class Variables
There are two ways to access Class Variables – through the class name or through an object reference. When you access a Class Variable through the class name, you don’t need to create an object of the class.
You can access it directly through the class. When you access a Class Variable through an object reference, you need to create an object of the class.
You can then access the Class Variable through the object reference. Here’s an example that illustrates how to create and access Class Variables:
class Car:
wheels = 4 #Class Variable
def __init__(self, make, model):
self.make = make #Instance Variable
self.model = model #Instance Variable
def get_car_details(self):
print("Make:", self.make)
print("Model:", self.model)
print("Number of Wheels:", Car.wheels)
toyota = Car("Toyota", "Corolla")
honda = Car("Honda", "Civic")
toyota.get_car_details() #Make: Toyota, Model: Corolla, Number of Wheels: 4
honda.get_car_details() #Make: Honda, Model: Civic, Number of Wheels: 4
In the above example, we created a class called ‘Car’ with a Class Variable called ‘wheels.’ We initialized ‘wheels’ to 4 as all cars usually have four wheels.
We then created two instances of the ‘Car’ class – ‘toyota’ and ‘honda.’ We accessed the Class Variable ‘wheels’ through the class name ‘Car’ in the instance method ‘get_car_details()’, and it printed the same value (4) for both instances.
Conclusion
Class Variables in Python are essential for maintaining the state and behavior of a class. They are shared by all instances of a class and can be accessed through the class name or an object reference.
Instance variables, on the other hand, hold data that is unique to each instance of a class. Inheritance plays an important role when it comes to Class Variables.
Subclasses inherit Class Variables from the superclass, and the subclass can override the superclass’s Class Variable. We hope this article clarified everything you needed to know about Class Variables in Python.
Happy coding!
Modifying Class Variables
In the previous section, we learned about creating and accessing Class Variables in Python. In this section, we will learn about modifying Class Variables.
Modifying Class Variables using class name
To modify a Class Variable using the class name, simply assign a new value to the Class Variable using the class name. All instances of the class will have the new value for the Class Variable.
Here’s an example:
class Car:
color = "red" #Class Variable
print(Car.color) #Output: red
Car.color = "blue"
print(Car.color) #Output: blue
In the above example, we created a Class Variable called ‘color’ and assigned it the value ‘red.’ We accessed the Class Variable using the class name ‘Car.’ Then we changed the value of ‘color’ to ‘blue’ using the class name ‘Car,’ and all instances of the class will have the new value for the Class Variable.
Modifying Class Variables using object reference
To modify a Class Variable using an object reference, simply assign a new value to the Class Variable using the object reference. This will modify the Class Variable for that particular instance of the class.
Here’s an example:
class Car:
color = "red" #Class Variable
car1 = Car()
car2 = Car()
print(car1.color) #Output: red
print(car2.color) #Output: red
car1.color = "blue"
print(car1.color) #Output: blue
print(car2.color) #Output: red
In the above example, we created two objects/instances of the class ‘Car’ named ‘car1’ and ‘car2.’ Initially, both objects had the same value for the Class Variable ‘color.’ Then we changed the value of ‘color’ to ‘blue’ using the object reference ‘car1,’ and the value of ‘color’ for ‘car1’ changed. But ‘car2’ stayed the same.
Wrong Use of Class Variables
One common mistake newcomers to Python might make is Shadowing. Shadowing occurs when you assign a new value to a Class Variable within a method or function of the same name.
Here’s an example:
class Car:
color = "red"
def __init__(self, color):
self.color = color #Instance variable
def get_color(self):
return self.color
car1 = Car("blue")
print(car1.color) #Output: blue
print(car1.get_color()) #Output: blue
In the above example, we created a Class Variable called ‘color’ and assigned it the value ‘red.’ Then we created an instance variable called ‘color’ in the __init__() method, which takes a parameter called ‘color’ and assigns it to the instance variable ‘color.’ This instance variable ‘color’ shadows the Class Variable ‘color.’ In the ‘get_color()’ method, we returned the instance variable ‘color.’ When we called the ‘get_color()’ method using ‘car1.get_color(),’ it returned the instance variable ‘color’ instead of the Class Variable ‘color.’
Examples of Class Variables
Example 1: Basic Class Variable Creation and Accessing
In this example, we will create a Class Variable called ‘count’ and use it in the class method ‘get_count()’ to return the value of ‘count.’ We will then create two instances of the class ‘Person’ and access the Class Variable ‘count’ using the class name ‘Person.’
class Person:
count = 0 #Class Variable
def __init__(self, name):
self.name = name
Person.count += 1
def get_count(self):
return Person.count
person1 = Person("John")
person2 = Person("Mary")
print(Person.count) #Output: 2
print(person1.get_count()) #Output: 2
print(person2.get_count()) #Output: 2
In the above example, we use the Class Variable ‘count’ to maintain a count of the number of instances of the class ‘Person’ created. We initialized the Class Variable ‘count’ to zero.
Then we created the __init__() method with a parameter called ‘name,’ which takes the name of the person and initializes the instance variable ‘name.’ We then incremented the Class Variable ‘count’ by one every time an instance of the class is created. Finally, we created two instances of the class ‘Person’ – ‘person1’ and ‘person2’ – and accessed the Class Variable ‘count’ using the class name ‘Person.’
Example 2: Modifying Class Variables and Object Reference
In this example, we will create a Class Variable called ‘price’ and use it in the instance method ‘change_price()’ to modify the value of ‘price’ using an object reference.
class Product:
price = 100 #Class Variable
def change_price(self, new_price):
self.price = new_price
product1 = Product()
product2 = Product()
print(product1.price) #Output: 100
print(product2.price) #Output: 100
product1.change_price(200)
print(product1.price) #Output: 200
print(product2.price) #Output: 100
In the above example, we created a Class Variable called ‘price’ and initialized it to 100. Then we created an instance method called ‘change_price()’ that takes a parameter called ‘new_price’ and assigns it to the instance variable ‘price.’ We then created two objects of the class ‘Product’ – ‘product1’ and ‘product2.’ Initially, both objects had the same value for the Class Variable ‘price.’ Then we modified the value of ‘price’ for ‘product1’ by calling the method ‘change_price()’ and passing an argument 200.
When we printed the value of ‘price’ for both objects, ‘product1’ had a new value of 200, and ‘product2’ remained the same at 100.
Example 3: Inheritance and Changing Class Variable in Child Class
In this example, we will create a parent class called ‘Shape’ with a Class Variable called ‘color.’ We will create a child class called ‘Square’ that inherits from the parent class ‘Shape.’ We will then override the Class Variable ‘color’ in the child class ‘Square.’
class Shape:
color = "red" #Class Variable
class Square(Shape):
color = "blue" #Overriding Class Variable in Child Class
square = Square()
print(square.color) #Output: blue
In the above example, we created a parent class called ‘Shape’ with a Class Variable called ‘color,’ which is initialized to ‘red.’ We then created a child class called ‘Square’ that overrides the value of ‘color’ to ‘blue.’ We then created an instance of the ‘Square’ class called ‘square’ and accessed the value of ‘color.’ The value of ‘color’ for the object ‘square’ is ‘blue’ because we overrode the Class Variable ‘color’ in the child class.
Conclusion
In this article, we covered creating, accessing, modifying, and wrong use of Class Variables in Python. We also provided examples of creating, modifying, and overriding Class Variables in child classes.
Understanding the concept of Class Variables is essential for writing efficient and clear code in Python. We hope that this article has provided a comprehensive understanding of Class Variables in Python.
Happy coding!
In this article, we have covered everything you need to know about Class Variables in Python. We started by defining what a Class Variable is and its purpose.
We then compared and contrasted Class Variables with Instance Variables, and we explored how Class Variables behave in Inheritance. Next, we proceeded to explain how to create and access Class Variables, and we provided examples of modifying Class Variables using the class name and object reference.
Additionally, we mentioned Shadowing as a common mistake when using Class Variables. Finally, we provided examples of Class Variables to illustrate their usage.
Class variables are an integral part of object-oriented programming. They are a convenient way of sharing data between objects of the same class.
Class variables are shared between all objects of a class, as opposed to instance variables, which are unique to each object.
When it comes to Inheritance, Class Variables can be inherited by child classes like any other member of the parent class.
However, the child class can override the Class Variable in question to suit its needs. This behavior is useful when working with complex class hierarchies, where the subclasses share certain attributes with the superclass.
Creating Class Variables is straightforward. They are declared within the class body but outside of any method.
The Class Variables value can be initialized at the same time as declaration or later in the class definition.
Accessing Class Variables can be achieved in two ways: through the class name or through an object reference. When accessing a Class Variable through the class name, there is no need to create an object of the class.
You can access it directly through the class. When accessing a Class Variable through an object reference, you need to create an object of the class.
You can then access the Class Variable through the object reference.
Modifying Class Variables is easy. Class variables can be modified using either the class name or an object reference.
If you modify a Class Variable using the class name, the new value is shared among all objects of that class. On the other hand, when using an object reference to modify a Class Variable, only the targeted object’s Class Variable will be modified.
However, it is possible to misuse Class Variables, leading to a common mistake known as Shadowing. Shadowing is when you assign a new value to a Class Variable within an instance method or function of the same name.
This action creates an instance variable, which gets priority over the Class Variable. This can lead to unexpected results when trying to access the Class Variable.
Finally, we provided examples of Class Variables to illustrate their usage. In the first example, we created a Class Variable to maintain a count of the number of instances of a class created.
The next example showed how to modify Class Variables using an object reference. In the final example, we demonstrated how to override a Class Variable in a child class.
In conclusion, Class Variables are a powerful tool in Python when used correctly. They are useful in sharing data between multiple objects of the same class and maintaining attributes of the class as a whole.
Misusing Class Variables can lead to unexpected results, so it is vital to understand their purpose and how they behave in different contexts. With a good understanding of Class Variables, you can create efficient and robust Python programs.
In this article, we covered the concept of Class Variables in Python programming. We defined Class Variables and explained how they differ from Instance Variables.
We also covered their behavior in Inheritance, how to create and access them, and provided examples of modifying them. We also cautioned against misusing Class Variables, specifically Shadowing, and provided examples of their usage.
Understanding Class Variables is essential for writing efficient and clear code in Python, especially when dealing with complex class hierarchies. Overall, this article highlights the importance of Class Variables and their significance in programming.