code: def _feature_indices(self, all_index_groups, primary_key, feature_df):
          
          # each feature only gets indices if they're *not* the
          # primary key and have non-null values in the feature's
          # subset of data
          result = []
          for index_group in all_index_groups:
              # is the index group just a primary key?
              if len(index_group) == 1 and index_group[0] == primary_key:
                  continue
              index_column_values = feature_df[index_group]
              if len(index_column_values.dropna()) == 0:
                  continue
              result.append(index_group)
          return result
docs: Choose subset of index group tuples from `all_index_groups` which are
          applicable to a particular feature (not same as its primary key, have
          non-null values).