def get_longest_string(in_list):
    """
    Get the longest string(s) in a list.

    :param in_list: list of strings
    :return: single string if there's only one with the max length,
             or a list of strings if there are several.
    """
    if len(in_list) == 0:
        return None
    max_length = max(len(x) for x in in_list)
    matches = [x for x in in_list if len(x) == max_length]
    if len(matches) == 1:
        return matches[0]
    else:
        return matches