Tkinter nested mainloop

Question

I am writing a videoplayer with tkinter/python, so basically I have a GUI that can play a video. Now, I would like to implement a stop button, meaning that I would have a mainloop() for the GUI, and another nested mainloop() to play/stop the video and return to the GUI startup window. Here, it is said:

Event loops can be nested; it's ok to call mainloop from within an event handler.

However, I do not understand how to implement this nesting. Can someone please provide me with a simple example of such a script?

EDIT

Here is a working version of my code, since it seems that I am doing something exotic. Of course, I am a newbie, so it may be that I am mistaken about the necessity to have a nested mainloop.

#!/usr/bin/python

import numpy as np
from multiprocessing import Process, Queue
import cv2
import cv2.cv as cv
from PIL import Image, ImageTk
import Tkinter as tk

def image_capture(queue):
    vidFile = cv2.VideoCapture(0)
    while True:
        flag, frame=vidFile.read()
        frame = cv2.cvtColor(frame,cv2.cv.CV_BGR2RGB)
        queue.put(frame)
        cv2.waitKey(10)

def update_all(root, imagelabel, queue, process, var):
    if var.get()==True:
        im = queue.get()
        a = Image.fromarray(im)
        b = ImageTk.PhotoImage(image=a)
        imagelabel.configure(image=b)
        imagelabel._image_cache = b  # avoid garbage collection
        root.update()
        root.after(0, func=lambda: update_all(root, imagelabel, queue, process, var))
    else:
        print var.get()
        root.quit()

def playvideo(root, imagelabel, queue, var):
    print 'beginning'
    p = Process(target=image_capture, args=(task,))
    p.start()
    update_all(root, imagelabel, queue, p, var)
    print 'entering nested mainloop'
    root.mainloop()
    p.terminate()
    if var.get()==False:
        im = ImageTk.PhotoImage(file='logo.png')
        imagelabel.configure(image=im)
        imagelabel._image_cache = im  # avoid garbage collection
        root.update()
    var.set(True)
    print 'finishing'

if __name__ == '__main__':
    #initialize multiprocessing
    task = Queue()
    #GUI of root window
    root = tk.Tk()
    #the image container
    image_label = tk.Label(master=root)
    image_label.grid(column=0, row=0, columnspan=2, rowspan=1)
    #fill label with image until video is loaded
    bg_im = ImageTk.PhotoImage(file='logo.png')
    image_label['image'] = bg_im
    #frame for buttons
    button_frame = tk.Frame(root)
    button_frame.grid(column=0, row=1, columnspan=1)
    #load video button and a switch to wait for the videopath to be chosen
    load_button = tk.Button(master=button_frame, text='Load video',command=lambda: playvideo(root, image_label, task, switch))
    load_button.grid(column=0, row=0, sticky='ew')
    #Stop button
    switch = tk.BooleanVar(master=root, value=True, name='switch')
    stop_button = tk.Button(master=button_frame, text='Stop',command=lambda: switch.set(False))
    stop_button.grid(column=0, row=1, sticky='ew')
    #quit button
    quit_button = tk.Button(master=button_frame, text='Quit',command=root.destroy)
    quit_button.grid(column=0, row=2, sticky='ew')
    root.mainloop()

Answer 1

This should be an example of a nested mainloop in Tkinter:

import Tkinter

def main():
    print 'main'
    t.mainloop()
    print 'end main'

t = Tkinter.Tk()
b = Tkinter.Button(t, command = main)
b.pack()
t.mainloop()

Whenever you hit the button a new mainloop is executed.

main
main
main
main
main
# now close the window
end main
end main
end main
end main
end main