i experienced some problem when override method _amount_all(), 

here the error:

TypeError: _amount_all() got an unexpected keyword argument 'context'

and here my code:

...

    @api.multi
    def _amount_all(self, field_name):
        reslt = {}
        result = super(sale_order, self)._amount_all(field_name)
        for orders in self.browser(self.id):
            reslt[orders.id] = {
                'discount_fidelity': self.discount_fidelity
            }
            result[orders.id]['discount_fidelity'] = cur_obj.round(
                reslt[orders.id]['discount_fidelity'])
            result[orders.id]['amount_total'] = result[orders.id][
                'amount_total'] - reslt[orders.id]['discount_fidelity']
        return result

The point why i override this function because i want to add new field to use when counting total amount.

Thanks.