2D vector graphics Problem

[Karl Max]

Hi all,

I'm new here. Name's Max from tuscany, and I don't speak english very well 
:-) I am not veteran at coding, I am most like an "artisan coder" since 
commodore 64 times.

Now the problem:
I would like to have a little 2d engine to calculate and trace segments and 
poly's and their collisions, rotations and trasformations. I didn't 
understand too much in tutorials (you're all high school coders, well 
informed in maths and programming theory!!! :-))) so I took my old school 
books on Cartesian space and had a refresh. Proceeding in this not too 
optimized way of coding my needs, I took down some generic classes to 
describe primitive objects in space (vertexes, segments, poly's etc...).

Now I try to make a point rotate around another point.
Here's my piece of code:

def rotate(self, w):
 # This method belongs to the class Vertex
 # w = angle expressed in radiants
 x, y = self.coords
 xRel, yRel = self.relPoint # The rotation should be relative to this
 sin, cos = math.sin(w), math.cos(w)
 x = x * cos - y * sin - xRel * cos + yRel * sin + xRel
 y = x * sin + y * cos - xRel * sin - yRel * cos + yRel
 self.coords = (x,y)

I know the style isn't professional, and if you should have some piece of 
advice, you're welcome... but that's not the question.
When I render it graphically, using pygame 1.6, the point tends to fall 
towards the center of rotation round after round. I mean if I have a loop 
rotating the point by a fixed angle, the point is like "attracted" by the 
center every round it does, like in a vortex.
I think it depends from float representation inside python, but I have no 
clear idea about how to resolve it. Maybe should I use Decimal module? Or is 
there any trick on this subject (2D vector graphics?) that I can't even 
imagine?
Thanks you all,
 Max