def _get_descriptors(self):
        """Returns three elements tuple with socket descriptors ready
        for gevent.select.select
        """
        rlist = []
        wlist = []
        xlist = []

        for socket, flags in self.sockets.items():
            if isinstance(socket, zmq.Socket):
                rlist.append(socket.getsockopt(zmq.FD))
                continue
            elif isinstance(socket, int):
                fd = socket
            elif hasattr(socket, 'fileno'):
                try:
                    fd = int(socket.fileno())
                except:
                    raise ValueError('fileno() must return an valid integer fd')
            else:
                raise TypeError('Socket must be a 0MQ socket, an integer fd '
                                'or have a fileno() method: %r' % socket)

            if flags & zmq.POLLIN:
                rlist.append(fd)
            if flags & zmq.POLLOUT:
                wlist.append(fd)
            if flags & zmq.POLLERR:
                xlist.append(fd)

        return (rlist, wlist, xlist)
Returns three elements tuple with socket descriptors ready
        for gevent.select.select