我是Python和程序设计的新手，但似乎无法理解为什么此函数不更新全局变量

    global weight
    weight = 'value'
    def GetLiveWeight():
        SetPort()
        while interupt == False:
            port.write(requestChar2)
            liveRaw = port.read(9)
            liveRaw += port.read(port.inWaiting())
            time.sleep(0.2)
            weight = liveRaw.translate(None, string.letters)
        return weight

我也尝试过

    weight = 'value'
    def GetLiveWeight():
        global weight
        SetPort()
        while interupt == False:
            port.write(requestChar2)
            liveRaw = port.read(9)
            liveRaw += port.read(port.inWaiting())
            time.sleep(0.2)
            weight = liveRaw.translate(None, string.letters)
        return weight

    try:
        threading.Thread(target = GetLiveWeight).start()
        print liveWeight
    except:
        print "Error: unable to start thread"