Example 1

               class res_city(models.Model):
                    _name = 'res.city'

                    name = fields.Char('Name')
                    state_id = fields.Many2one('res.country.state', 'State')


                    #and put onchange where you use state and city like:

                    @api.onchange('state_id')
                    def _onchange_state_id(self):
                        if self.state_id:
                            res = {'domain': {'city_id': [('state_id', '=', self.state_id.id)]}}

    

Example 2

<pre> <field name="state_id" domain=[('country_id', '=', country_id)]"/> @api.onchange('country_id') def _onchange_country_id_wrapper(self): res = {'domain': {'state_id': []}} if self.country_id: res['domain']['state_id'] = [('country_id', '=', self.country_id.id)] return res