def process_invoice(self):
    self = self.with_user(SUPERUSER_ID)
    order_ids = self.order_ids.filtered(lambda order: order.state == 'draft')
    for order in order_ids:
        order.action_confirm()
        moves = order._create_invoices(final=False)
        for move in moves:
            new_quantity = 1
            new_price_unit = 200
            for line in move.invoice_line_ids:
                line.update(line._get_price_total_and_subtotal(quantity=new_quantity,price_unit=new_price_unit))
                price_subtotal = line._get_price_total_and_subtotal(quantity=new_quantity,price_unit= new_price_unit).get('price_subtotal')
                line.update(line._get_fields_onchange_balance(quantity=new_quantity, price_subtotal=price_subtotal, force_computation=True))

How to use 

_get_price_total_and_subtotal

and

_get_fields_onchange_balance

to write 'quantity' or 'price_unit'?

Thanks