Polymorphism

Polymorphism means calling the SAME method name on different types and having each one do its own type-appropriate thing — you already saw the mechanism in the last module (Square inheriting Rectangle.area), but the real power shows up when you have a MIX of types and treat them uniformly:

class Shape:
    def area(self):
        raise NotImplementedError

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):
        import math
        return math.pi * self.r ** 2

Sometimes you need to know an object's ACTUAL type at runtime — not to change behavior (that's what polymorphism already handles automatically via .area()), but for things like logging or display. type(obj).__name__ gives you the class name as a string:

for s in shapes:
    print(f"{type(s).__name__}: {s.area():.2f}")
# Rectangle: 6.00
# Circle: 3.14
# Rectangle: 16.00

A common beginner mistake is reaching for if isinstance(s, Rectangle): ... elif isinstance(s, Circle): ... to handle each type differently — that defeats the entire point of polymorphism. If you find yourself writing that kind of type-checking chain, it's usually a sign the logic belongs inside each class's own method instead.