from greenlet import greenlet
from time import sleep

def ping():
    while True:
        print "ping"
        sleep(0.5)
        pong_greenlet.switch()

def pong():
    while True:
        print "pong"
        sleep(0.5)
        ping_greenlet.switch()

ping_greenlet = greenlet(ping)
pong_greenlet = greenlet(pong)

ping_greenlet.switch()