def requiredUnlessTable(col_name, arg, dm, df, con=None):
    """
    Col_name must be present in df unless
    arg (table_name) is present in contribution
    """
    table_name = arg
    if col_name in df.columns:
        return None
    elif not con:
        return None
    elif table_name in con.tables:
        return None
    else:
        return "{} column is required unless table {} is present".format(col_name, table_name)