message: Speedup the type conversion
diff: @@ -82,6 +82,9 @@ def _merge_and_transform_series(src_series_list, col, dense_cols,
  return [result_series, min_max]
  +def _convert_to_string(series):
  + return series.astype(str)
  +
  def _merge_columns_and_feature_cross(series_list, min_max, feature_pairs,
  feature_cross):
  name_to_series = dict()
  @@ -188,7 +191,24 @@ def preprocess(src_txt_name, dst_txt_name, normalize_dense, feature_cross):
  logging.info('_convert_to_string')
  - df = pd.DataFrame(df.values.astype(str))
  + futures = dict()
  + for col in cols:
  + future = executor.submit(_convert_to_string, df[col])
  + futures[col] = future
  + if feature_cross != 0:
  + for pair in feature_pairs:
  + col = pair[0] + '_' + pair[1]
  + future = executor.submit(_convert_to_string, df[col])
  + futures[col] = future
  +
  + logging.info('_store_to_df')
  + for col, future in futures.items():
  + ret = future.result()
  + try:
  + df[col] = ret
  + except:
  + print_exc()
  + futures = dict()
  logging.info('write to a CSV file')
  df.to_csv(dst_txt_name, sep=' ', header=False, index=False)
  