<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def requiredUnless(col_name, arg, dm, df, *args):
 """ Each string will be a column name. Col_name is required in df unless each column from arg is present. """

    # if column name is present, no need to check if it is required
    if col_name in df.columns:
        return None
    arg_list = arg.split(",")
    arg_list = [argument.strip('"') for argument in arg_list]
    msg = ""
    for a in arg_list:
        # ignore validations that reference a different table
        if "." in a:
            continue
        if a not in df.columns:
            msg += "{} column is required unless {} is present.  ".format(col_name, a)
    if msg:
        return msg
    else:
        return None
    return None