I have yet to read the whole PEP, and I haven't written any serious Python, either, but I find this to be very well written. It uses a simple example of a "Cir

I have yet to read the whole PEP, and I haven't written any serious Python, either, but I find this to be very well written. It uses a simple example of a "Circle" class, a subclass of "Shape": class Shape: def set_scale(self, scale: float) -> Shape: self.scale = scale return self Shape().set_scale(0.5) # => Shape class Circle(Shape): def set_radius(self, r: float) -> Circle: self.radius = r return self Circle().set_scale(0.5) # *Shape*, not Circle Circle().set_scale(0.5).set_radius(2.7) # => Error: Shape has no attribute set_radius The error occurs because right now, Python cannot see that `self` is a `Circle`. This PEP fixes that.