def _write_local_data_files(self, cursor):
        """
        Takes a cursor, and writes results to a local file.

        :return: A dictionary where keys are filenames to be used as object
            names in GCS, and values are file handles to local files that
            contain the data for the GCS objects.
        """
        schema = list(map(lambda schema_tuple: schema_tuple[0], cursor.description))
        col_type_dict = self._get_col_type_dict()
        file_no = 0
        tmp_file_handle = NamedTemporaryFile(delete=True)
        if self.export_format == 'csv':
            file_mime_type = 'text/csv'
        else:
            file_mime_type = 'application/json'
        files_to_upload = [{
            'file_name': self.filename.format(file_no),
            'file_handle': tmp_file_handle,
            'file_mime_type': file_mime_type
        }]

        if self.export_format == 'csv':
            csv_writer = self._configure_csv_file(tmp_file_handle, schema)

        for row in cursor:
            # Convert datetime objects to utc seconds, and decimals to floats.
            # Convert binary type object to string encoded with base64.
            row = self._convert_types(schema, col_type_dict, row)

            if self.export_format == 'csv':
                csv_writer.writerow(row)
            else:
                row_dict = dict(zip(schema, row))

                # TODO validate that row isn't > 2MB. BQ enforces a hard row size of 2MB.
                s = json.dumps(row_dict, sort_keys=True).encode('utf-8')
                tmp_file_handle.write(s)

                # Append newline to make dumps BigQuery compatible.
                tmp_file_handle.write(b'\n')

            # Stop if the file exceeds the file size limit.
            if tmp_file_handle.tell() >= self.approx_max_file_size_bytes:
                file_no += 1
                tmp_file_handle = NamedTemporaryFile(delete=True)
                files_to_upload.append({
                    'file_name': self.filename.format(file_no),
                    'file_handle': tmp_file_handle,
                    'file_mime_type': file_mime_type
                })

                if self.export_format == 'csv':
                    csv_writer = self._configure_csv_file(tmp_file_handle, schema)

        return files_to_upload
Takes a cursor, and writes results to a local file.

        :return: A dictionary where keys are filenames to be used as object
            names in GCS, and values are file handles to local files that
            contain the data for the GCS objects.