docs: def save_feature_list(self, obj, set_id, feature_list_id):
          """
          Pickle the specified feature list to a file.
          Example: `save_feature_list(project, X_tfidf_train, 'train', 'tfidf')`.
  
          Args:
              obj: The object to pickle (e.g., a numpy array or a Pandas dataframe)
              project: An instance of pygoose project.
              set_id: The id of the subset (e.g., 'train' or 'test')
              feature_list_id: The name for this feature list.
          """
  
          save(obj, self.features_dir + 'X_{}_{}.pickle'.format(set_id, feature_list_id))