def get_diff(df, column1, column2):
    """Get the difference between two column values.

        Args:
            df: Pandas DataFrame.
            column1: First column.
            column2: Second column.

        Returns:
            Value of summed columns.

        Usage:
            df['item_quantity_vs_mean'] = get_diff(df, 'item_quantity', 'item_code_item_quantity_mean')
        """
    return df[column1] - df[column2]