Return complete proxy name associated with a hostport of a given
        ``proxy_address``. If ``proxy_address`` is unkonwn or empty,
        return None.

def get_proxy(self, proxy_address):
        """
        Return complete proxy name associated with a hostport of a given
        ``proxy_address``. If ``proxy_address`` is unkonwn or empty,
        return None.
        """
        if not proxy_address:
            return None
        hostport = extract_proxy_hostport(proxy_address)
        return self.proxies_by_hostport.get(hostport, None)