Python allows us to create methods that can be used as if they were attributes. This gives us very pleasant syntax for getting a derived value from an object. A method that appears to be an attribute is called a property. We'll extend our Point class with two more methods:

    @property
    def r(self):
        return math.sqrt(self.x**2 + self.y**2)
    @property
    def θ(self):
        return math.atan2(self.y, self.x)

We've defined two functions using the @property decorator. This decorator can be used with a function that has only the instance variable, self, as a parameter.

Here's how we can use these properties:

>>> p = Point(12, 5)
>>> round(p.r, 1)
13.0
>>> round(math.degrees(p.θ), 1)
22.6

We've accessed these methods as if they were simple attributes of the object, p. Using p.r and p.θ can be more pleasant than having to write p.r() and p.θ() in a complex formula. The preceding properties are explicitly read-only. We get an exception if we...