python中的线程不会并行发生

python中的线程不会并行发生

要并行获取多个网址，一次最多限制20个连接：

import urllib2from multiprocessing.dummy import Pooldef generate_urls(): # generate some dummy urls    for i in range(100):        yield 'http://example.com?param=%d' % idef get_url(url):    try: return url, urllib2.urlopen(url).read(), None    except EnvironmentError as e:         return url, None, epool = Pool(20) # limit number of concurrent connectionsfor url, result, error in pool.imap_unordered(get_url, generate_urls()):    if error is None:       print result,