import threading
from time import sleep

# res: max number of resources. If changed to 1, it functions
# identically to a mutex/lock object
res = 2
sema = threading.Semaphore(res)

class res_thread(threading.Thread):
    def run(self):
        global res
        n = self.getName()
        for i in range(1, 4):
            # acquire a resource if available and work hard
            # for 2 seconds.  if all res are occupied, block
            # and wait
            sema.acquire()
            res = res - 1
            print n, "+  res count", res
            sleep(2)

                        # after done with resource, return it to pool and flag so
            res = res + 1
            print n, "-  res count", res
            sema.release()

# create 4 threads, each acquire resorce and work
for i in range(1, 5):
    t = res_thread()
    t.start()