async def _on_address_status(self, addr, status):
        """Handle the change of the status of an address.
        Should remove addr from self._handling_addr_statuses when done.
        """
        raise NotImplementedError()  # implemented by subclasses
Handle the change of the status of an address.
        Should remove addr from self._handling_addr_statuses when done.
        
electrum/synchronizer.py