I'm new to OpenERP 7, what is the correct way to return a tree view? How can i return the data i obtained from my function? Here's my class code:

class albaranes_fallidos(osv.osv):
_name = 'albaranes_fallidos'
_inherit = 'stock.picking'
_columns = {}
def find(self, cr, uid, ids, context=None):
    stock_picking       = self.pool.get('stock.picking')
    exist_state         = stock_picking.search(cr, uid, [('state','=','confirmed'),('origin','ilike','SO')])
    ###
    exist_lines         = stock_picking.browse(cr, uid, exist_state, context)
    ###
    exist_IN            = stock_picking.search(cr, uid, [('type','=','in'),('origin','ilike',':SO')])
    for SO in exist_lines:
        exist_PO            = stock_picking.browse(cr, uid, exist_IN, context)
        for POSO in exist_PO:
            if SO.origin in POSO.origin:
                _logger.info(POSO.origin)
                break
Thanks in advance.