I need to override the action_button_confirm method  from the sales.order model, but i can't make the inheritance work. It seems that this code is never called during execution. 

I'ḿ using odoo  8.0. The module is installed and depends on sale module. 

# -*- coding: utf-8 -*-
from openerp import models, fields, api 
import logging 
_logger =logging.getLogger(__name__)

class sales_warning(models.Model):
    _inherit = "sale.order"
@api.multi   
def action_button_confirm(self):
    _logger.info("begin overwritten action_button_confirm()")
    res = super(sales_warning, self).action_button_confirm()            
    return res