Problem with override function module: sale_layout

Hi, ubu 10.4, oerp 6.0.2, new db with sale_layout installed:
I see that function _amount_line is write in sale/sale.py module and sale_layout override it in sale_layout/sale_layout.py file
I put a pdb.set_trace() in both the function and I see that when I click compute botton in order form the method that is called is the sale one, no work on sale_layout

sale.py function:
class sale_order_line(osv.osv):
    def _amount_line(self, cr, uid, ids, field_name, arg, context=None):
        tax_obj = self.pool.get('account.tax')
        cur_obj = self.pool.get('res.currency')
        res = {}
        if context is None:
            context = {}
        for line in self.browse(cr, uid, ids, context=context):
            price = line.price_unit * (1 - (line.discount or 0.0) / 100.0)
            taxes = tax_obj.compute_all(cr, uid, line.tax_id, price, line.product_uom_qty, line.order_id.partner_invoice_id.id, line.product_id, line.order_id.partner_id)
            cur = line.order_id.pricelist_id.currency_id
            res[line.id] = cur_obj.round(cr, uid, cur, taxes['total'])
        return res

sale_layout function: <<<< here openerp don't pass!!!
class sale_order_line(osv.osv)
    def _amount_line(self, cr, uid, ids, field_name, arg, context=None):
        res = {}
        for line in self.browse(cr, uid, ids, context=context):
            if line.layout_type == 'article':
                return super(sale_order_line, self)._amount_line(cr, uid, ids, field_name, arg, context)
        return res