@staticmethod
def _whois_filter(sources, src, whoismodule, whoismodule_cache):
    """
        Help method for filtering sources by abuse group's networks
        """
    if src not in whoismodule_cache:
        whoismodule_cache[src] = bool(whoismodule.lookup(src))
    if whoismodule_cache[src]:
        sources.add(src)
    return sources
