I have a combobox defined in res.partner:

 payment_method = fields.Selection(
                selection=[
                        ('bank_transfer', 'Transferencia Bancaria'),
                        ('check', 'Cheque'),   
                        ('direct_debit', 'Domiciliación')
                ],
                string='Método de pago')

I have the same combobox defined in account.invoice.

Then I have a method redefined in account.invoice to set the combobox to the value set in res.partner when partner is selected:

        @api.multi
        def onchange_partner_id(self, type, partner_id, date_invoice=False, payment_term=False, partner_bank_id=False, company_id=False):
                payment_method = ***RETRIEVE payment_method FROM partner_id and assign to payment_method in account.invoice****
                return super(account_invoice, self).onchange_partner_id(type, partner_id, date_invoice=date_invoice, payment_term=payment_term, partner_bank_id=partner_bank_id, company_id=company_id)

How can I retrieve the value payment_method from res.partner and assign to account.invoice form payment_method?

Both combos appears fine in both forms (res.partner and account.invoice) and value at res.partner can be stored correctly, I just want to programatically assign the value from res.partner to account.invoice form.