def split_part_key(key):
    """
    Split the standard part key.

    Only returns port if present in key.

    Parameters
    ----------
    key : str
        Standard part key as hpn:rev

    Returns
    -------
    tuple
        hpn, rev, [,port]

    """
    split_key = key.split(":")

    if len(split_key) == 2:
        return split_key[0], split_key[1]

    return split_key[0], split_key[1], split_key[2]