Python：从Gevent Greenlet获得价值

我正在学习Gevent,但无法获得greenlet中调用的函数返回的值.以下代码：

import gevent.monkey
gevent.monkey.patch_socket()

import gevent
from gevent import Greenlet

import urllib2
import simplejson as json

def fetch(pid):
    response = urllib2.urlopen('http://time.jsontest.com')
    result = response.read()
    json_result = json.loads(result)
    datetime = json_result['time']

    print('Process %s: %s' % (pid,datetime))
    return json_result['time']

def synchronous():
    for i in range(1,10):
        fetch(i)

def asynchronous():
    threads = [Greenlet.spawn(fetch,i) for i in range(10)]
    result = gevent.joinall(threads)
    print [Greenlet.value(thread) for thread in threads]

print('Synchronous:')
synchronous()

print('Asynchronous:')
asynchronous()

给我错误：

print [Greenlet.value(thread) for thread in threads]
AttributeError: type object 'Greenlet' has no attribute 'value'

我做错了什么,如何从每个greenlet中获取价值？