code: def _drop_collections(self, df):
          
          all_cols = df.columns
          keep_cols = []
  
          # Check whether each column contains collections.
          for c in all_cols:
              if len(self._col_type_set(c, df).intersection([set, dict, list])) == 0:
                  keep_cols.append(c)
          return df[keep_cols]
docs: Drops columns containing collections (i.e. sets, dicts, lists) from a DataFrame.
  
          :param pandas.DataFrame df: Usually self._data.
          :return: pandas.DataFrame