You can try:

def action_view_po(self, cr, uid, ids, context=None):

mod_obj = self.pool.get('ir.model.data')

purchase_obj = self.pool.get('purchase.order')

list_po = []

for po in purchase_obj.search(cr, uid, [('sale_id', '=', ids[0])], context=context):

list_po.append(po)

action_model, action_id = tuple(mod_obj.get_object_reference(cr, uid, 'purchase', 'purchase_form_action'))

action = self.pool.get(action_model).read(cr, uid, action_id, context=context)

ctx = eval(action['context'])

action.update({'context': ctx,

'domain': "[('id','in',%s)]" % list_po})

return action

It is the name of the class of the window that i want to open.can you please give me an example

'res_model': 'add_credit'   <= this seems a bit strange...