gevent semaphore example (from http://sdiehl.github.io/gevent-tutorial/)

gevent_semaphore.py

# from http://sdiehl.github.io/gevent-tutorial/
 
from gevent import sleep
from gevent.pool import Pool
from gevent.coros import BoundedSemaphore
 
sem = BoundedSemaphore(4)  # only allows 4 greenlets at one time, others must wait until one is released
 
def worker1(n):
    sem.acquire()
    print('Worker %i acquired semaphore' % n)
    sleep(0)
    sem.release()
    print('Worker %i released semaphore' % n)
 
def worker2(n):
    with sem:
        print('Worker %i acquired semaphore' % n)
        sleep(0)
    print('Worker %i released semaphore' % n)
 
pool = Pool()
pool.map(worker1, xrange(1,5))
pool.map(worker2, xrange(15,25))