def get_ical_file_name(zip_file):
    """Gets the name of the ical file within the zip file."""
    ical_file_names = zip_file.namelist()
    if len(ical_file_names) != 1:
        raise Exception(
            "ZIP archive had %i files; expected 1."
            % len(ical_file_names)
        )
    return ical_file_names[0]