synchronization primitive: semaphore example (resource limitation)

semaphore.py

#!/usr/bin/env python
# -*- coding: utf8 -*-
                                                                                                                  
import threading
import time
import random

sema = threading.Semaphore(2)

class Grab(threading.Thread):
    def __init__(self, url):
        threading.Thread.__init__(self)
        self.url = url
    
    def run(self):
        try:
            k = random.randint(10, 20)
            print "fetch page: %s, waiting for: %ds" % (self.url, k)
            time.sleep(k)
            print "page %s done" % self.url
        finally:
            sema.release()

class Crawler(threading.Thread):                                                                                  
    def __init__(self, hosts):
        threading.Thread.__init__(self)
        self.hosts = hosts

    def run(self):
        for url in self.hosts:
            sema.acquire()
            g = Grab(url)
            g.start()

print "Starting Crawling ..."
hosts = ['www.google.com', 'www.baidu.com', 'www.163.com', 'www.sina.com.cn', 'www.sohu.com', 'www.tencent.com']
c = Crawler(hosts)
c.start()
c.join()