Class Methods

Class methods are defined on a class and can be invoked using the class name or an instance. They are declared with the @classmethod decorator and typically take cls as the first parameter, representing the clas itself. These methods can manipulate class variables but cannot access instance-specific data.

Syntax

class ExampleClass:
    @classmethod
    def example_class_method(cls, param1, param2):
        # Implementation code

Key Characteristics

• Callable via class or instance: Class methods can be executed by referencing the class directly or through an instance.
• First parameter is the class: The initial argument, conventionally named cls, refers to the class.
• No instance variable access: Class methods operate on class-level data and cannot interact with instance attributes.

Example Usage

class Employee:
    company = "TechCorp"  # Class variable

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

    @classmethod
    def update_company(cls, new_company):
        cls.company = new_company

# Invoke via class
Employee.update_company("InnovateInc")
print(Employee.company)  # Output: InnovateInc

# Invoke via instance
emp = Employee("Bob", 101)
emp.update_company("FutureTech")
print(Employee.company)  # Output: FutureTech

Class Variables

Class variables are defined within a class but outside any methods. They are shared across all instances and can be accessed or modified using the class name or an instance. Their value remains consistent unless explicitly altered.

Syntax

class ExampleClass:
    shared_attribute = initial_value

Key Characteristics

• Shared across instances: All instance reference the same class variable.
• Accessible via class or instance: Both the class and its instances can read or write to class variables.
• Instance variable precedence: If an instance defines an attribute with the same name, it overrides the class variable for that instance.

Example Usage

class Employee:
    company = "TechCorp"  # Class variable

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

# Access via class
print(Employee.company)  # Output: TechCorp

# Access via instance
emp = Employee("Alice", 102)
print(emp.company)  # Output: TechCorp

# Modify class variable
Employee.company = "GlobalTech"
print(Employee.company)  # Output: GlobalTech
print(emp.company)  # Output: GlobalTech

# Override with instance variable
emp.company = "LocalBiz"
print(emp.company)  # Output: LocalBiz
print(Employee.company)  # Output: GlobalTech

Integrated Example

This example demonstrates the interaction between class methods and class variables.

class Rectangle:
    default_color = "blue"  # Class variable

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

    @classmethod
    def set_default_color(cls, color):
        cls.default_color = color

    def area(self):
        return self.width * self.height

    def describe(self):
        return f"A {self.default_color} rectangle with area {self.area()}"

# Create instances
rect1 = Rectangle(4, 5)
rect2 = Rectangle(3, 7)

# Display descriptions
print(rect1.describe())  # Output: A blue rectangle with area 20
print(rect2.describe())  # Output: A blue rectangle with area 21

# Update class variable
Rectangle.set_default_color("green")

# Updated descriptions
print(rect1.describe())  # Output: A green rectangle with area 20
print(rect2.describe())  # Output: A green rectangle with area 21