code: def append(self, data):
          
          if isinstance(data, six.text_type):
              self._message.add_body_data(data.encode(self._encoding))
          elif isinstance(data, six.binary_type):
              self._message.add_body_data(data)
docs: Append a section to the body.
  
          :param data: The data to append.
          :type data: str or bytes