def must_replace_suffix(str, suffix, replacement):
    """
    Replaces the given suffix in the string. If the string does not have the suffix, a runtime
    error will be raised.
    """
    splits = str.rsplit(suffix, maxsplit=1)
    if len(splits) != 2 or splits[1]:
        raise RuntimeError(str + " does not contain " + suffix)
    return splits[0] + replacement