you can try to override the write function instead it will be run on create or any time you edit the record it will compute the field like:

def write(self, vals): 

if condition:

    vals['field1']=somevalue 

res = super(classname, self).write(vals)  

return res
or 

you can use an onchange compute method
like:
field1=fields.Integer(compute="get_value")

@api.onchange('partner_id')
def get_value(self):
field1=somevalue

Thank you for this answer.

I thinked about it but the problem is that the field stay empty until i click on save button

I prefer that the field is filled  before, otherwise user will think that something is wrong.