This is pretty straight forward but you need to do a couple of things (assuming OpenERP 7)

1 Create your new module.

2 Create a class that inherits osv.Model

3 Add an _inherit = 'xxxxxxx'

4 Implement your new dispatch_rpc function.

5 Install your module and it should work.

A typical pattern is to call super and then process the results.

class MyModel(osv.Model):

_inherit = 'sale.order.line'

def product_id_change(self, cr, uid, ids, ...):

res = super(MyModel, self).product_id_change(cr, uid, ids...)

# do stuff with res.

return res

Here is an other useful link:

Regards.