8.2 Attributes that depend on each other

Attributes of an instance can be changed (or created) by simply assigning them a value. However, if other attributes depend on the one just changed, it is desirable to change them simultaneously.

To demonstrate this, we consider an example: let's define a class that defines an object for planar triangles from three given points. A first attempt to set up such a class could be as follows:

class Triangle:
    def __init__(self,  A, B, C):
        self.A = array(A)
        self.B = array(B)
        self.C = array(C)
        self.a = self.C - self.B
        self.b = self.C - self.A
        self.c = self.B - self.A
    def area(self):
        return abs(cross(self.b, self.c)) / 2

An instance of this triangle is created by this:

tr = Triangle([0., 0.], [1., 0.], [0., 1.])

Then its area is computed by calling the corresponding method:

tr.area() # returns 0.5

If we change an attribute, say point B, the corresponding edges a and c are not automatically...