Abstract Base Classes

About

An Abstract Base Class (ABC) in Python is a class that defines a blueprint for other classes. It is not intended to be instantiated directly but instead serves as a base class for other classes. Abstract Base Classes define methods that must be implemented by any concrete (non-abstract) subclass, ensuring that subclasses adhere to a specific interface or contract

Python provides abc module to implement ABC

Key Principles

• Cannot Be Instantiated

  We cannot create objects of an abstract class. It is only used as a blueprint for other classes

• Abstract Methods

  A method declared in an ABC that has no implementation in the ABC itself. Subclasses must override and implement these methods

• Optional Concrete Methods

  ABCs can also define concrete methods (methods with implementations) that can be inherited by subclasses

• Ensures Interface Compliance

  ABCs ensure that all subclasses implement the required methods, providing a form of compile-time check in Python's dynamic environment

Example

from abc import ABC, abstractmethod

# Define an abstract base class
class Shape(ABC):
    @abstractmethod
    def area(self):
        pass

    @abstractmethod
    def perimeter(self):
        pass


# Concrete subclass must implement the abstract methods
class Rectangle(Shape):
    def __init__(self, width, height):
        self.width = width
        self.height = height

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

    def perimeter(self):
        return 2 * (self.width + self.height)


# Instantiate concrete subclass
rect = Rectangle(10, 5)
print(f"Area: {rect.area()}")  # Area: 50
print(f"Perimeter: {rect.perimeter()}")  # Perimeter: 30
print(isinstance(rect, Shape))  # True
print(issubclass(Rectangle, Shape))  # True

Use Cases

• Interface Enforcement
• Improved Design
• Polymorphism

Pros

• Encapsulation of Contracts
• Code maintainability
• Error Prevention