退出tk窗口时线程不会关闭

I have an application written in Python, using Tkinter. One of the features allows a serial port to be opened, after which any messages received over the serial port are displayed in a text window. This works fine. The problem comes when I close the window, which doesn't kill the thread monitoring the serial port. It then has to be killed manually (alternatively, unplugging the USB-serial cable causes an exception which kills the process).I have an application written in Python, using