为什么第二次线程就不能用了？

Question

我是一个python的外行，我正在创建这个下载器应用程序，使用的是 tkinter和urllib.request 我建立了一个没有任何线程的程序，当我下载一个文件时，它会冻结，直到文件被下载，我不希望这样，所以我尝试了 线程和事件 而且它似乎工作得很好，但当我想下载第二个文件时，它却不能工作。为什么会出现这种情况？我做错了什么？我的代码。

from tkinter import *
from tkinter import font as tkFont
import random
import urllib.request
import requests
import threading
import wget
import queue


def printsth():
    print("Yay it works! ")


def main_menu():
    root = Tk()
    root.title('8-bit downloader ')
    root.iconbitmap(r"C:\Users\rayanravesh\PycharmProjects\GUI_Calculator\icon.ico")
    root.geometry("600x300")
    # the top menu
    global num, chum, var
    num = IntVar()
    chum = IntVar()
    var = IntVar()
    menu = Menu(root)
    root.config(menu=menu)
    submenu = Menu(menu)
    menu.add_cascade(label="Settings", menu=submenu)

    def custom_op():
        custom = Toplevel()
        custom.iconbitmap(r"C:\Users\rayanravesh\PycharmProjects\GUI_Calculator\icon.ico")
    submenu.add_command(label="Customization ", command=custom_op)

    def settings_op():
        global gps
        set_win = Toplevel()
        set_win.iconbitmap(r"C:\Users\rayanravesh\PycharmProjects\GUI_Calculator\icon.ico")
        path_label = Label(set_win, text="Current default download path: ")
        path_entry = Entry(set_win, width=30)
        file_read = open('Data.txt', 'r')
        data_base = file_read.read()
        path_entry.insert(0, data_base)
        file_read.close()

        def default_output():
            global location
            file_read2 = open('Data.txt', 'r+')
            file_read2.truncate(0)
            file_read2.close()
            write_file2 = open('Data.txt', 'w')
            write_file2.write(path_entry.get())
            write_file2.close()
            location = path_entry.get() + "\\"
            default_location = location.replace("\\", "\\\\")
        path_btn = Button(set_win, text="Submit ", command=default_output)
        path_label.pack(anchor=CENTER, expand=1)
        path_entry.pack(anchor=CENTER, expand=1)
        path_btn.pack(anchor=CENTER, expand=1)
    submenu.add_command(label="Settings ", command=settings_op)
    submenu.add_separator()
    submenu.add_command(label="Exit", command=root.destroy)

    # the section menu
    editmenu = Menu(menu)
    menu.add_cascade(label="Sections(soon)", menu=editmenu)
    editmenu.add_command(label="Downloader", command=printsth)
    editmenu.add_command(label="Converter", command=printsth)
    editmenu.add_command(label="Media Player", command=printsth)
    editmenu.add_command(label="Editor", command=printsth)
    # the tool bar
    toolbar = Frame(root, bg="light gray")
    insert_button = Button(toolbar, text="Insert an image", command=printsth)
    insert_button.pack(side=LEFT, padx=2, pady=2)
    print_button = Button(toolbar, text="Print", command=printsth)
    print_button.pack(side=LEFT, padx=2, pady=2)
    toolbar.pack(side=TOP, fill=X)

    # the download initiation
    def initiate():
        event.set()

    # the status bar
    status_bar = Label(root, text="Downloading...", bd=1, relief=SUNKEN, anchor=W)
    status_bar.pack(side=BOTTOM, fill=X)

    # the download frame
    body_frame = Frame(root, bg="light blue")
    download_button = Button(body_frame, text="Download! ", command=initiate, border=3, width=20, height=5)
    download_design = tkFont.Font(size=12, slant='italic')
    download_button['font'] = download_design
    download_button.pack(side=LEFT, pady=5, padx=5)
    body_frame.pack(side=LEFT, fill=Y)
    # the main interaction menu
    inter_frame = Frame(root)
    global name_entry, format_entry, url_entry, output_entry
    url_entry = Entry(inter_frame, width=30)
    label = Label(inter_frame, text="Enter the image URL: ")
    file_format = Label(inter_frame, text="Choose your file format: ")
    format_entry = Entry(inter_frame, width=30)
    file_name = Label(inter_frame, text="File's name: ")
    name_entry = Entry(inter_frame, width=30)
    check_name = Checkbutton(inter_frame, text="Give a random name", variable=num)
    check_format = Checkbutton(inter_frame, text="Download with default format", variable=var)
    check_default = Checkbutton(inter_frame, text="Download to default path", variable=chum)
    check_default.deselect()
    output_path = Label(inter_frame, text="Choose output path: ")
    output_entry = Entry(inter_frame, width=30)
    file_name.pack(anchor=CENTER, expand=1)
    name_entry.pack(anchor=CENTER, expand=1)
    check_name.pack(anchor=CENTER, expand=1)
    label.pack(anchor=CENTER, expand=1)
    url_entry.pack(anchor=CENTER, expand=1)
    file_format.pack(anchor=CENTER, expand=1)
    format_entry.pack(anchor=CENTER, expand=1)
    format_entry.insert(0, '.')
    check_format.pack(anchor=CENTER)
    output_path.pack(anchor=CENTER, expand=1)
    output_entry.pack(anchor=CENTER, expand=1)
    check_default.pack(anchor=CENTER, expand=1)
    inter_frame.pack(expand=1)
    root.mainloop()

    # the end!


def download_image():
    event.wait()
    while event.is_set():
        print('hi')
        global formatname
        if num.get() == 1:
            name = random.randrange(1, 1000000)
        else:
            name = str(name_entry.get())
        formatname = str(format_entry.get())
        '''if var.get() == 1:
            operator = str(url_entry.get())
            formatname = '.' + operator[-3] + operator[-2] + operator[-1]
        else:
            pass'''
        fullname = str(name) + formatname
        url = str(url_entry.get())
        fw = open('file-size.txt', 'w')
        file_size = int(requests.head(url, headers='accept-encoding': '').headers['Content-Length'])
        fw.write(str(file_size))
        fw.close()
        path = str(output_entry.get()) + "\\"
        if chum.get() == 1:
            filee = open('Data.txt', 'r')
            destination = filee.read()
            path = destination + "\\"
            output_entry.insert(0, destination)
            filee.close()
        else:
            output_entry.delete(0, END)
        urllib.request.urlretrieve(url, path.replace("\\", "\\\\") + fullname)


event = threading.Event()
t1 = threading.Thread(target=main_menu)
t2 = threading.Thread(target=download_image)
t1.start()
t2.start()