Newbie: question regarding references and class relationships

[Rui Maciel]

Let:
- class Point be a data type which is used to define points in space
- class Line be a data type which possesses an aggregate relationship with 
objects of type Point
- class Model be a container class which stores collections of Point and 
Line objects


Essentially, a Model object stores lists of Point objects and Line objects, 
and Line objects include references to Point objects which represent the 
starting and ending point of a line.

To reflect this class relationship, I've defined the following classes:

<code>
class Point:
        position = []

        def __init__(self, x, y, z = 0):
                self.position = [x, y, z]

class Line:
        points = ()
        def __init__(self, p_i, p_f):
                self.points = (p_i, p_f)

class Model:
        points = []
        lines = []


</code>

It would be nice if, whenever a Point object was updated, the Line objects 
which are associated with it could reflect those updates directly in 
Line.p_i and Line.p_f.

What's the Python way of achieving the same effect?


Thanks in advance,
Rui Maciel