Python3, Tkinter - display message after process completion

If the process blocks the main loop while it is executed, you can just create a method in your main class that runs the process and shows the messagebox:

import tkinter as tk
from tkinter import messagebox
import time


class BackendProcess:
    def __init__(self):
        self.finished = False

    def run(self):
        time.sleep(10)
        self.finished = True


class GUI(tk.Tk):
    def __init__(self):
        tk.Tk.__init__(self)
        self.process = BackendProcess()
        tk.Button(self, text='Run', command=self.run).pack()

    def run(self):
        self.process.run() 
        messagebox.showinfo('Info', 'Process completed!')


if __name__ == '__main__':
    gui = GUI()
    gui.mainloop()

If the process does not block the main loop, then the above method does not work and the messagebox is displayed right after the start of the process. To avoid this, the after method can be used to periodically check the finished flag of the process:

import tkinter as tk
from tkinter import messagebox
import time
import threading


class BackendProcess:
    def __init__(self):
        self.finished = False

    def task(self):
        time.sleep(2)
        print('finished')
        self.finished = True

    def run(self):
        thread = threading.Thread(target=self.task, daemon=True)
        thread.start()


class GUI(tk.Tk):
    def __init__(self):
        tk.Tk.__init__(self)
        self.process = BackendProcess()
        tk.Button(self, text='Run', command=self.run).pack()

    def run(self):
        self.process.run()
        self.check_process()

    def check_process(self):
        """ Check every 1000 ms whether the process is finished """
        if self.process.finished:
            messagebox.showinfo('Info', 'Process completed')
        else:
            self.after(1000, self.check_process)


if __name__ == '__main__':
    gui = GUI()
    gui.mainloop()

From the comments, I think the first method should work since the process is executed in the main thread. I have shown the second case anyway because completion notification are especially useful for time-consuming tasks that are often executed in a separated thread to avoid freezing the GUI.