code: def add_metadata(self, key, value):
          
          self._metadata[key] = value
  
          # write the current metadata to file
          json.dump(self._metadata, open(self.metadata_filename, 'w'),
                    indent=JSON_INDENT,
                    sort_keys=True)
docs: Adds metadata (key-value pairs) to the dataset.
  
          Parameters
          ----------
          key : str
              name for metadata
          value : :obj:`object` must be JSON serializable
              content of metadata