def append_id(expr, id_col='append_id'):
    """
    Append an ID column to current column to form a new DataFrame.

    :param str id_col: name of appended ID field.

    :return: DataFrame with ID field
    :rtype: DataFrame
    """
    if hasattr(expr, '_xflow_append_id'):
        return expr._xflow_append_id(id_col)
    else:
        return _append_id(expr, id_col)
Append an ID column to current column to form a new DataFrame.

    :param str id_col: name of appended ID field.

    :return: DataFrame with ID field
    :rtype: DataFrame