102. Instance Variables vs Class Variables

1. Basic Example of Instance and Class Variables

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class Person:
    species = "Human"  # Class variable

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

person1 = Person("John", 25)
person2 = Person("Alice", 30)

print(person1.name, person1.species)  # Output: John Human
print(person2.name, person2.species)  # Output: Alice Human

Explanation: species is a class variable shared by all instances, while name and age are instance variables, specific to each instance.

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2. Changing Instance and Class Variables

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class Animal:
    species = "Mammal"  # Class variable

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

animal1 = Animal("Lion")
animal2 = Animal("Elephant")

animal1.species = "Big Cat"  # Modifying instance variable

print(animal1.species)  # Output: Big Cat
print(animal2.species)  # Output: Mammal (unchanged)

Explanation: Modifying species for animal1 creates an instance variable specific to that object, not affecting other instances.

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3. Class Variable Shared Across Instances

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class Car:
    wheels = 4  # Class variable

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

car1 = Car("Sedan")
car2 = Car("SUV")

print(car1.wheels)  # Output: 4
print(car2.wheels)  # Output: 4

# Changing class variable
Car.wheels = 3
print(car1.wheels)  # Output: 3
print(car2.wheels)  # Output: 3

Explanation: The class variable wheels is shared by all instances of the class, and changing it affects all instances.

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4. Class and Instance Variables with Same Name

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class Person:
    type = "Human"  # Class variable

    def __init__(self, name, type="Homo sapiens"):
        self.name = name  # Instance variable
        self.type = type  # Instance variable (overrides class variable)

person = Person("John")
print(person.type)  # Output: Homo sapiens (instance variable)
print(Person.type)  # Output: Human (class variable)

Explanation: The instance variable type overrides the class variable type within the instance, but the class variable remains unchanged.

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5. Accessing Class Variables via Class and Instance

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class Product:
    category = "Electronics"  # Class variable

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

product = Product("Laptop", 1200)
print(product.category)  # Output: Electronics (accessed through instance)
print(Product.category)  # Output: Electronics (accessed through class)

Explanation: Both the class and instances can access class variables, but accessing them via an instance is allowed, though less recommended.

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6. Modifying Instance Variable Without Affecting Class Variable

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class Book:
    genre = "Fiction"  # Class variable

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

book1 = Book("1984", "George Orwell")
book2 = Book("Brave New World", "Aldous Huxley")

book1.genre = "Dystopian"  # Modifying instance variable

print(book1.genre)  # Output: Dystopian
print(book2.genre)  # Output: Fiction (class variable unchanged)

Explanation: Modifying the genre attribute for book1 only changes its instance variable and does not affect book2.

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7. Instance Variables in Inherited Classes

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class Animal:
    sound = "Roar"  # Class variable

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

class Lion(Animal):
    def __init__(self, name, region):
        super().__init__(name)
        self.region = region  # Instance variable

lion = Lion("Simba", "Africa")
print(lion.name)  # Output: Simba
print(lion.region)  # Output: Africa
print(Lion.sound)  # Output: Roar (inherited class variable)

Explanation: The inherited sound class variable is shared among all Lion instances, while the name and region are instance variables.

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8. Overriding Class Variables in Subclasses

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class Animal:
    species = "Mammal"  # Class variable

class Dog(Animal):
    species = "Canine"  # Overriding class variable

dog = Dog()
print(dog.species)  # Output: Canine
print(Animal.species)  # Output: Mammal

Explanation: Dog overrides the species class variable, and this change only applies to instances of Dog, not the parent Animal.

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9. Class Variable Affects All Instances

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class Counter:
    count = 0  # Class variable

    def __init__(self):
        Counter.count += 1

counter1 = Counter()
counter2 = Counter()

print(counter1.count)  # Output: 2
print(counter2.count)  # Output: 2

Explanation: The class variable count is shared across all instances, so both counter1 and counter2 reflect the same value.

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10. Instance Variable Default Value

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class Employee:
    company = "TechCorp"  # Class variable

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

employee1 = Employee("Alice", "Manager")
employee2 = Employee("Bob", "Developer")

employee1.position = "Senior Manager"  # Changing instance variable

print(employee1.position)  # Output: Senior Manager
print(employee2.position)  # Output: Developer

Explanation: The position is an instance variable that can be modified independently for each instance.

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Summary of Differences:

• Instance Variables are specific to each object created from a class. They are set using self in the __init__ method.

• Class Variables are shared across all instances of the class and are set directly in the class body, not using self.