Help designing reading/writing a xml-fileformat

[Jacob Kroon]

I'm writing a block-diagram editor, and could use some tips about 
writing/reading
diagrams to/from an xml file format. The basic layout of my code :

class Diagram {
    Blocks blocks[]
}

class Block {
    int x, y
}

class Square(Block) {
    int width, height
}

class Circle(Block) {
    int radius
}

I'd like to be able to output something similar to this:

<diagram>
    <square x='12' y='34' width='50' height='40'/>
    <circle x='40' y='60' radius='20/>
</diagram>

One idea I had was to make Diagram and Block derive from 
xml.dom.minidom.Element,
and then somehow catch when I set an attribute, so that setAttribute() 
gets called with correct
arguments. Perhaps then I could just do a simple diagram.toprettyxml().

Anyone have a good idea on how to approach this problem ?
(I do not want to use the pickle module)

/Jacob Kroon