first im a novice in python

here is the code im using 

@api.multi

    def _prepare_invoice(self):

        invoice_vals = super(SaleOrder, self)._prepare_invoice()

        invoice_vals = {

'field1' : self.field1 ,

'field2' : self.field2

}

but it got me an account error

when i use only 

invoice_vals['field1'] =self.field1

it works perfectly but i dont know how to add other field since i have 5 in total