Inheritance and Polymorphism

class Shape:
    def area(self):
        raise NotImplementedError

    def describe(self):
        return f"{type(self).__name__} with area {self.area():.2f}"


class Rectangle(Shape):
    def __init__(self, w, h):
        self.w, self.h = w, h

    def area(self):
        return self.w * self.h


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

    def area(self):
        return 3.14159 * self.r ** 2


for shape in [Rectangle(2, 3), Circle(1)]:
    print(shape.describe())   # polymorphism: same call, different behaviour

class Square(Rectangle):
    def __init__(self, side):
        super().__init__(side, side)

class Engine:
    def start(self):
        return "vroom"

class Car:
    def __init__(self):
        self.engine = Engine()   # a Car has an Engine

    def start(self):
        return self.engine.start()