create a new field x_po_plus_x_sz

@api.onchange('x_po','x_sz')   

def getsum_x_po_x_sz(self):
        if self.x_po and self.x_sz:
            self.x_po_plus_x_sz=self.x_po + self.x_sz

You need to make a compute field for the third one with an onchange annotationthirdfield = fields.Float(compute='_compute_sum', string="SUM")

@api.onchange('x_po','x_sz')

def _compute_sum(self): 

        self.thirdfield = self.x_po + self.x_sz

Hi Tomasz

You need to add a calculated field that compute the calculation and if you wanna make it more dynamically you could add also an onchange for the source field for the sum to provide the result without the need of saving the form

Read more about it in the docs

https://www.odoo.com/documentation/10.0/reference/orm.html#fields

hello,

thank you for answers; I had hope it is to be done inside form view - but I guess code as above is to be added somewhere else, am I right? if yes - sorry for asking - where to insert code as above:

---

@api.one

def _get_total(self):

try:

self.thirdfield = self.first + self.second

thirdfield = fields.Float(compute='_get_total', string="")

---

regards

Tomasz