Day 13 进程和线程

应用案例1 的代码：为什么class DownloadTaskHandler 和 Download()函数必须要在main函数里面？
`
import time
import tkinter
import tkinter.messagebox
from threading import Thread

def main():

class DownloadTaskHandler(Thread):

    def run(self):
        time.sleep(10)
        tkinter.messagebox.showinfo('提示', '下载完成!')
        # 启用下载按钮
        button1.config(state=tkinter.NORMAL)

def download():
    # 禁用下载按钮
    button1.config(state=tkinter.DISABLED)
    # 通过daemon参数将线程设置为守护线程(主程序退出就不再保留执行)
    # 在线程中处理耗时间的下载任务
    DownloadTaskHandler(daemon=True).start()

def show_about():
    tkinter.messagebox.showinfo('关于', '作者: 骆昊(v1.0)')

top = tkinter.Tk()
top.title('单线程')
top.geometry('200x150')
top.wm_attributes('-topmost', 1)

panel = tkinter.Frame(top)
button1 = tkinter.Button(panel, text='下载', command=download)
button1.pack(side='left')
button2 = tkinter.Button(panel, text='关于', command=show_about)
button2.pack(side='right')
panel.pack(side='bottom')

tkinter.mainloop()

if name == 'main':
main()
`

Start with the embedded example in issue #387 and trace main(), DownloadTaskHandler, and download() together with Python threading and tkinter references. Determine how the nested definitions relate to the GUI callbacks and document whether that structure is required; done means the scope question is answered clearly for readers of Day 13.