code: def _write_current_buffer_for_group_key(self, key):
          
          write_info = self.write_buffer.pack_buffer(key)
          self.write(write_info.get('file_path'),
                     self.write_buffer.grouping_info[key]['membership'])
          self.write_buffer.clean_tmp_files(write_info)
          self.write_buffer.add_new_buffer_for_group(key)
docs: Find the buffer for a given group key, prepare it to be written
          and writes it calling write() method.