ZeroMQ: have to sleep before send

I'm write a zeromq demo with Forwarder device (with pyzmq)

Here are the codes(reference to https://learning-0mq-with-pyzmq.readthedocs.org/en/latest/pyzmq/devices/forwarder.html ):

forwarder.py

import zmqcontext = zmq.Context()frontend = context.socket(zmq.SUB)frontend.bind('tcp://*:5559')frontend.setsockopt(zmq.SUBSCRIBE, '')backend = context.socket(zmq.PUB)backend.bind('tcp://*:5560')zmq.device(zmq.FORWARDER, frontend, backend)

sub.py

import zmqcontext = zmq.Context()socket = context.socket(zmq.SUB)socket.connect('tcp://localhost:5560')socket.setsockopt(zmq.SUBSCRIBE, '')while True:    print socket.recv()

pub.py

import zmq, timecontext = zmq.Context()socket = context.socket(zmq.PUB)socket.connect('tcp://localhost:5559')# time.sleep(0.01)socket.send('9 hahah')

I run python forwarder.py, python sub.py in the terminal

then run python pub.py, the subscriber can't get the message. However, if I sleep a little time(for example 0.01s) before send, it works.

So my problem is, why have I sleep before send? thanks.