Python class variables: syntax and examples

Python class variables (class variables in English) represent a structured way of sharing data between different instances of a class. Additionally, changes to a class variable are reflected in all instances of the class. Thus, they do not need to be redefined at each instantiation.

Python class variables: what are they?

In Python, class variables are created within a class. Unlike instance variables, which are specific to each instance of a class, class variables keep the same value for all instances. These variables are usually defined before the constructor and are accessible to all instances of the class.

Python class variables are often used to store information common to all instances of a classsuch as configuration values, default values, or common states in a class hierarchy. Additionally, class variables can be used to define methods or class attributes that are shared by all instances of a class, improving functionality and code organization.

Python class variables: syntax

Python class variables are created inside the class definition and outside of methods.

class Car:
    total_cars = 0  # Class variable to track the total number of cars
    def __init__(self, brand, model):
        self.brand = brand  # Instance variable for the car brand
        self.model = model  # Instance variable for the car model
        Car.total_cars += 1  # Increment the total number of cars upon each instantiation
    def display_details(self):
        print(f"Brand: {self.brand}, Model: {self.model}")
car1 = Car("BMW", "X3")
car2 = Car("Audi", "A4")
# Accessing class variable and instance variables
print(f"Total number of cars: {Car.total_cars}")  # Output: 2
car1.display_details()  # Output: Brand: BMW, Model: X3
car2.display_details()  # Output: Brand: Audi, Model: A4

Inside the classroom Carwe define the class variable total_cars to track the total number of cars. The manufacturer __init__ is called when a new instance of the class is created. At each instantiation, the instance variables brand And model are defined and the class variable total_cars is increased by 1.

By calling the method display_details()we display the details of each car instance. In a f-Stringa Python string formatting method; we access the class variable total_cars to display the total number of cars created.

Examples of using Python class variables

For safe use of class variables in Python, it is mainly access, modification and inheritance that play a role.

Accessing class variables

Access to Python class variables is done by the name of the class or instancefollowed by a period and the name of the variable.

class MyClass:
    class_var = "This is a class variable"
# using class name to access class variable
print(MyClass.class_var)  # Output: This is a class variable
# using instance to access class variable
obj = MyClass()
print(obj.class_var)  # Output: This is a class variable

In Python, class variables can be retrieved from different places:

• In the constructor : the class variable can be called inside the constructor, either by the keyword selfeither by the name of the class.
• In instance methods : the class variable can be called inside instance methods either by the keyword self either by the name of the class.
• Outside the classroom : outside the class, the class variable can be accessed either by the object reference or by the class name.

Modify class variables

Python class variables can be modified directly by the class name. If an instance variable has the same name as a class variable and you try to modify the variable via the instance, a new instance variable will be created and the class variable will remain unchanged.

class MyClass:
    class_var = "Original class variable"
obj = MyClass()
print(obj.class_var)  # Output: Original class variable
MyClass.class_var = "Modified class variable"
print(obj.class_var)  # Output: Modified class variable

In this example we replace the class variable class_var by assigning it a new value via the class MyClass. The modification is automatically taken into account by the instance obj.

Class Variables and Inheritance

Through inheritance, derived classes can access class variables of the base class and use them without having to redefine them.

class BaseClass:
    base_var = "Base variable"
class DerivedClass(BaseClass):
    pass
print(DerivedClass.base_var)  # Output: Base variable

Here we define a base class BaseClass which contains the class variable base_var. The derived class DerivedClass inherits from BaseClass. Thanks to the inheritance, DerivedClass can access the class variable base_var from the base class and use it without having to redefine it.