Updates the list with changes.

def save(self):
        """
        Updates the list with changes.
        """
        # Based on the documentation at
        # http://msdn.microsoft.com/en-us/library/lists.lists.updatelistitems%28v=office.12%29.aspx

        # Note, this ends up un-namespaced. SharePoint doesn't care about
        # namespaces on this XML node, and will bork if any of these elements
        # have a namespace prefix. Likewise Method and Field in
        # SharePointRow.get_batch_method().
        batches = E.Batch(ListVersion='1', OnError='Return')
        # Here's the root element of our SOAP request.
        xml = SP.UpdateListItems(SP.listName(self.id), SP.updates(batches))

        # rows_by_batch_id contains a mapping from new rows to their batch
        # IDs, so we can set their IDs when they are returned by SharePoint.
        rows_by_batch_id, batch_id = {}, 1

        for row in self._rows:
            batch = row.get_batch_method()
            if batch is None:
                continue
            # Add the batch ID
            batch.attrib['ID'] = text_type(batch_id)
            rows_by_batch_id[batch_id] = row
            batches.append(batch)
            batch_id += 1

        for row in self._deleted_rows:
            batch = E.Method(E.Field(text_type(row.id),
                                     Name='ID'),
                             ID=text_type(batch_id), Cmd='Delete')
            rows_by_batch_id[batch_id] = row
            batches.append(batch)
            batch_id += 1

        if len(batches) == 0:
            return

        response = self.opener.post_soap(LIST_WEBSERVICE, xml,
                                         soapaction='http://schemas.microsoft.com/sharepoint/soap/UpdateListItems')

        for result in response.xpath('.//sp:Result', namespaces=namespaces):
            batch_id, batch_result = result.attrib['ID'].split(',')
            row = rows_by_batch_id[int(batch_id)]

            error_code = result.find('sp:ErrorCode', namespaces=namespaces)
            error_text = result.find('sp:ErrorText', namespaces=namespaces)
            if error_code is not None and error_code.text != '0x00000000':
                raise UpdateFailedError(row, batch_result,
                                        error_code.text,
                                        error_text.text)

            if batch_result in ('Update', 'New'):
                row._update(result.xpath('z:row', namespaces=namespaces)[0],
                            clear=True)
            else:
                self._deleted_rows.remove(row)

        assert not self._deleted_rows
        assert not any(row._changed for row in self.rows)