code: def diff(
      df: DataFrame,
      columns: dict[str, str],
      periods: int = 1,
      axis: PandasAxis = PandasAxis.ROW,
  ) -> DataFrame:
      """
      Calculate row-by-row or column-by-column difference for select columns.
  
      :param df: DataFrame on which the diff will be based.
      :param columns: columns on which to perform diff, mapping source column to
             target column. For instance, `{'y': 'y'}` will replace the column `y` with
             the diff value in `y`, while `{'y': 'y2'}` will add a column `y2` based
             on diff values calculated from `y`, leaving the original column `y`
             unchanged.
      :param periods: periods to shift for calculating difference.
      :param axis: 0 for row, 1 for column. default 0.
      :return: DataFrame with diffed columns
      :raises InvalidPostProcessingError: If the request in incorrect
      """
      df_diff = df[columns.keys()]
      df_diff = df_diff.diff(periods=periods, axis=axis)
      return _append_columns(df, df_diff, columns)
  
imports: from pandas import DataFrame
  from superset.constants import PandasAxis
  from superset.utils.pandas_postprocessing.utils import _append_columns, validate_column_args
label_desc: Use image node_id 1 for calling a global function with example usage: diff(df, columns, periods, axis) and returns: _append_columns