Challenge 1: Shape Hierarchy — Possible Solution
====================================================================

class Shape:
    def area(self):
        return 0

class Square(Shape):
    def __init__(self, side):
        self.side = side

    def area(self):
        return self.side ** 2

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

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

shapes = [Square(4), Circle(3)]

for shape in shapes:
    print(shape.area())

Output:
16
28.27431

WHY THIS WORKS AS AN ANSWER
------------------------------
class Square(Shape) and class Circle(Shape) both reuse this chapter's
own class Dog(Animal) inheritance syntax — each becomes a subclass of
Shape, inheriting from it while adding its own __init__ and its own
area() method.

Both subclasses OVERRIDE Shape's area() (which just returns 0) with
their own correct formula — side ** 2 for a square, using the
exponent operator from Course 1, and 3.14159 * radius ** 2 for a
circle. This is the exact overriding pattern this chapter's Dog/speak()
example demonstrated, just applied to a calculation instead of a
returned string.

The for loop then reuses the chapter's own polymorphism example
directly: it calls shape.area() on each object without ever checking
"is this a Square or a Circle?" — each object automatically runs its
own class's version of area(), which is the entire point of
polymorphism demonstrated with the Dog/Cat speak() example earlier in
the chapter.