Tornado 学习——全局变量的疑惑

HI All:

这边初学习Tornado框架，写了以下代码：
g_ConnectionCount = 0

class MyTcpServer(TCPServer):
def handle_stream(self, stream, address):
print "New Connection"
g_ConnectionCount = g_ConnectionCount + 1
print "g_ConnectionCount",g_ConnectionCount
pass
pass

if __name__ == "__main__":
_myServer = MyTcpServer()
_myServer.listen(6666)
_ioLoop = IOLoop.instance()
tornado.autoreload.start(_ioLoop)
_ioLoop.start()
pass

本以为httpServer下是不行的，TcpServer下应该可行才对。但实际运行结果：UnboundLocalError: local variable 'g_ConnectionCount' referenced before assignment

Tornado 不是单进程单线程么？python下声明一个全局的g_ConnectionCount应该能被访问到才是。