def METHOD_NAME(self, asn: int) -> InternetExchange:
    """!
    @brief Get an existing InternetExchange.
    @param asn ASN of the IX.
    @returns InternetExchange.
    @throws AssertionError if ix does not exist.
    """
    assert asn in self.__ixes, "ix{} does not exist.".format(asn)
    return self.__ixes[asn]