from typing import Union
from typing import List


def comma_separated_string_to_list(line: str) -> Union[List[str], str]:
    """
    Converts a comma-separated string to a List of strings.
    If the input is a single item (no comma), it will be returned unchanged.
    """
    values = line.split(",")
    return values[0] if len(values) <= 1 else values