Class Variables in Python

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Class variables are variables that are shared among all instances of a class. They are defined within a class but outside any instance methods and are usually used to store data that should be the same across all instances.

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Defining Class Variables

Class variables are defined at the class level and can be accessed using either the class name or an instance.

class Car:
    # Class variable
    wheels = 4  

    def __init__(self, brand, color):
        self.brand = brand  # Instance variable
        self.color = color  # Instance variable

# Accessing class variable
print(Car.wheels)  # Output: 4

# Creating instances
car1 = Car("Toyota", "Red")
car2 = Car("Honda", "Blue")

# Accessing class variable via instance
print(car1.wheels)  # Output: 4
print(car2.wheels)  # Output: 4

Modifying Class Variables

1. Modifying at the Class Level

• Affects all instances.

Car.wheels = 6  # Changing the class variable

print(car1.wheels)  # Output: 6
print(car2.wheels)  # Output: 6

2. Modifying at the Instance Level

• Creates an instance-specific variable rather than modifying the class variable.

car1.wheels = 5  # This only changes for car1

print(car1.wheels)  # Output: 5
print(car2.wheels)  # Output: 6  (unchanged)

Accessing Class Variables

Class variables are usually accessed inside class methods using cls instead of self.

class Animal:
    species = "Mammal"  # Class variable

    def __init__(self, name):
        self.name = name  # Instance variable

    @classmethod
    def set_species(cls, new_species):
        cls.species = new_species  # Modifying class variable

# Accessing and modifying class variable
print(Animal.species)  # Output: Mammal

Animal.set_species("Reptile")
print(Animal.species)  # Output: Reptile

When to Use Class Variables?

1. When the data should be shared across all instances
2. When using constants or default values
3. When maintaining global counters or tracking objects