What are abstract classes and why are they needed?

An abstract class is a class that cannot be instantiated directly. It defines an interface (a set of mandatory methods) for its child classes.

The abc module:

from abc import ABC, abstractmethod

class Shape(ABC):
    @abstractmethod
    def area(self):
        """Calculate the area of the shape"""
        pass

    @abstractmethod
    def perimeter(self):
        """Calculate the perimeter of the shape"""
        pass

# shape = Shape()  # TypeError: Can't instantiate abstract class Create

Implementing an abstract class:

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)

class Circle(Shape):
    def __init__(self, radius):
        self.radius = radius

    def area(self):
        import math
        return math.pi * self.radius ** 2

    def perimeter(self):
        import math
        return 2 * math.pi * self.radius

rect = Rectangle(5, 3)
print(rect.area())       # 15
print(rect.perimeter())  # 16

Why they are needed:

• Contract: they guarantee that any child class implements all the required methods.
• Documentation: they clearly show what methods must be implemented.
• Early error detection: if an abstract method is missing in the child class, Python will raise a TypeError at instantiation, rather than failing later when the method is called.