如何捕捉@tornado.gen.coroutine里的异常

from tornado import gen
from tornado.ioloop import IOLoop

@gen.coroutine
def throw(a,b):
    try:
        a/b 
        raise gen.Return('hello')
    except Exception, e:
        pass
@gen.coroutine
def test():
    print "i'm ok"
    res = yield throw(1,1)
    print res #res始终为None
    print "here too"
                
test()          
IOLoop.instance().start()

res = yield throw(1,1)这里res获取的结果始终为空，因为throw内部用了try...except...，而@gen.coroutine本身就是以抛出异常的形式返回的，所以不管throw函数里的a/b这一句有不有异常，不管调用throw(1,0)还是throw(1,1)返回的都是None，
正确的做法是在外部调用的位置添加try...catch... 即：

@gen.coroutine
def throw(a,b):
    a/b 
    raise gen.Return('hello')

@gen.coroutine
def test():
    print "i'm ok"
    try:
        res = yield throw(1,0)
    except Exception, e:
        print 'EXCEPTION!', e
    print "here too"
                
test()          
IOLoop.instance().start()  

　　

参考：https://github.com/tornadoweb/tornado/issues/759