Python 3.6 open a message box using Tkinter and continue with the script

I have a script that runs a program and then begins a while True loop. I want to open a message box using easyGUI (that simplifies tkinter, so essentially, I want to open a message box using tkinter) and then enter into the while loop. When the message box is dismissed, I want to break from the loop and then kill the program.

My question is, how do I achieve this?

Current script (section of):

import subprocess, easygui
from time import sleep    

f = open("file.txt", "w")

notOpen = True

while True:
    if notOpen == True:
        subprocess.Popen("program.exe", shell=True, stdout=open(os.devnull, 'wb'))
        easygui.msgbox("\nRunning!", "FSRP engine")
        notOpen = False
    f.write("1=")
    f.flush()
    sleep(5)
    f.write("2=")
    f.flush()
    sleep(5)
    f.write("3")
    f.flush()
    sleep(5)

You just need to add None sector.

So your code would be:

import subprocess, easygui
from time import sleep    

f = open("file.txt", "w")

notOpen = True

while True:
    if notOpen == True:
        subprocess.Popen("program.exe", shell=True, stdout=open(os.devnull, 'wb'))
        if easygui.msgbox("\nRunning!", "FSRP engine") == 'OK':
            notOpen = False
        else:
            break
    f.write("1=")
    f.flush()
    sleep(5)
    f.write("2=")
    f.flush()
    sleep(5)
    f.write("3")
    f.flush()
    sleep(5)

Or:

import subprocess, easygui
from time import sleep    

f = open("file.txt", "w")

notOpen = True

while True:
    if notOpen == True:
        subprocess.Popen("program.exe", shell=True, stdout=open(os.devnull, 'wb'))
        if easygui.msgbox("\nRunning!", "FSRP engine") == 'OK':
            notOpen = False
        else:
            exit
    f.write("1=")
    f.flush()
    sleep(5)
    f.write("2=")
    f.flush()
    sleep(5)
    f.write("3")
    f.flush()
    sleep(5)