how to set domain with python in odoo

Is it possible to set domain on the python side without putting it in view? I tried to set the domain in the customer field (sale order) based on branch in the Customer menu/template

I have tried this but why it doesn't work ?

@api.onchange('partner_id')
def _onchange_cust_domain(self):
    for rec in self:
        if self.branch_id.id:
            cust_list = rec.partner_id.branch_id.id
        return {'domain' : {'partner_id' : [('branch_id', '=', cust_list)]}}

To set the partner_id domain when the branch_id field value changes, use the following onchange function:

Example:

@api.onchange('branch_id')
def onchange_branch_id(self):
    if self.branch_id:
        return {'domain': {'partner_id': ['|', ('branch_id', '=', False), ('branch_id', '=', self.branch_id.id)]}}

Odoo sets by default the following domain from the python code:

['|', ('company_id', '=', False), ('company_id', '=', company_id)]