def update_fee(self, tx: Optional[PartialTransaction]) -> None:
        """Updates self.fee_label. No other side-effects."""
        if self.is_reverse:
            sm = self.swap_manager
            fee = sm.get_fee_for_txbatcher()
        else:
            fee = tx.get_fee() if tx else None
        fee_text = self.window.format_amount(fee) + ' ' + self.window.base_unit() if fee else _("no input")
        self.fee_label.setText(fee_text)
        self.fee_label.repaint()  # macOS hack for #6269
Updates self.fee_label. No other side-effects.
electrum/gui/qt/swap_dialog.py