Python: Can't pop from an empty list

serial_list=[]
context = Context()
monitor = Monitor.from_netlink(context)
monitor.filter_by(subsystem='block',device_type='partition')
observer = GUDevMonitorObserver(monitor)
​
def device_connected(observer, device):
    Welcome.device_count+=1
​
    flag =False
    for iden in serial_list :
        if iden == device.__getitem__('ID_SERIAL_SHORT'):
            flag=True
​
    if flag ==False:
        serial_list.append(device.__getitem__('ID_SERIAL_SHORT'))
        Welcome.dev_label.append(str(device.__getitem__('ID_FS_LABEL')))
        size = len(Welcome.dev_label)
        label = gtk.Label('Device connected :: {0!r}'.format(Welcome.dev_label[size-1]))
        Welcome.vbox.pack_start(label)
        Welcome.window.show_all()
​
    if Welcome.device_count<2:
        label = gtk.Label('Connect the second device')
        Welcome.vbox.pack_start(label)
        Welcome.window.show_all()
​
    else :
        # 等待直到 Welcome.dev_label 列表中有元素后再弹出
        while len(Welcome.dev_label) == 0:
            pass
        Exchange()
​
observer.connect("device-added",device_connected)
monitor.start()
​
class Welcome:
    device_count = 0    
    window = gtk.Window()
    vbox= gtk.VBox(False, 5)
    dev_label = []
​
    def __init__(self):
​
        self.window.set_default_size(300, 300)
        self.window.set_title("Welcome")
​
        label = gtk.Label("Connect the desired device")
​
        self.vbox.pack_start(label)
        self.window.add(self.vbox)
​
        self.window.connect("destroy", lambda q: gtk.main_quit())
        self.window.show_all()
​
class Exchange:
    def __init__(self):
​
        # 确保在从 Welcome.dev_label 列表中弹出元素之前，列表中有元素
        while len(Welcome.dev_label) == 0:
            pass
        window1 = gtk.Window(Welcome.dev_label.pop())
        window2 = gtk.Window(Welcome.dev_label.pop())
​
        width = gtk.gdk.screen_get_default().get_width()
        height = gtk.gdk.screen_get_default().get_height()
​
        self.window1.resize(width/2,height)
        self.window2.resize(width/2,height)
​
        self.window2.move(self.window1.get_position()[0]+width/2, self.window1.get_position()[1])
​
        label = gtk.Label("Hello")
        self.window1.add(label)
​
        self.window1.connect("destroy" , lambda q : gtk.main_quit())
        self.window1.show_all()
​
        label = gtk.Label("World")
        self.window2.add(label)
​
        self.window2.connect("destroy",lambda q : gtk.main_quit())
        self.window2.show_all()
​
Welcome()
gtk.main()