Hi John Doe, try this:

from openerp.exceptions import UserError

res = self.search([('type','=',inv_type),('number','=',str(number_down)),('date_invoice','>',date_invoice)])
raise osv.except_osv(_('Title'),_(res))

or to see in your log:

import logging
_logger = logging.getLogger(__name__)

res = self.search([('type','=',inv_type),('number','=',str(number_down)),('date_invoice','>',date_invoice)])
logger.info('Content: %r',res)

I hope it helps you, regards.