Mastering Abstract Base Classes in Python: Concepts and Examples

• An Abstract Base Class or ABC mandates the derived classes to implement specific methods from the base class.

• It is not possible to create an object from a defined ABC class.

• Creating objects of derived classes is possible only when derived classes override existing functionality of all abstract methods defined in an ABC class.

ABC - Example

from abc import ABC, abstractmethod
class Shape(ABC):
    @abstractmethod
    def area(self):
        pass
    @abstractmethod
    def perimeter(self):
        pass

• In Example 1, Abstract base class Shape is defined with two abstract methods area and perimeter.

• With existing abstract class definition of Shape, if you try creating a Shape object it results in TypeError.

s1 = Shape()

Output

TypeError: Can't instantiate abstract class Shape with abstract methods area, perimeter

---

class Circle(Shape):
    def __init__(self, radius):
        self.__radius = radius
    @staticmethod
    def square(x):
        return x**2
    def area(self):
        return 3.14*self.square(self.__radius)
c1 = Circle(3.9)

• Creating object c1, with out definingperimeter inside derived class, Circle, resulted in TypeError.

Output

TypeError: Can't instantiate abstract class Circle with abstract methods perimeter

---

• Corrected class definition of Circle, that contains perimeter definition too.

class Circle(Shape):
    def __init__(self, radius):
        self.__radius = radius
    @staticmethod
    def square(x):
        return x**2
    def area(self):
        return 3.14*self.square(self.__radius)
    def perimeter(self):
        return 2*3.14*self.__radius
c1 = Circle(3.9)
print(c1.area())

Output

47.7594