Writing a Python UI for a seperate program with tkinter. The stop button for this program basically freezes the UI and continues with the script

If the other python script is made to run forever (has some kind of while True:), you can't run it on the command line as you did, because it will freeze your window while that script is running.

In order to run a python script on background you will need to do it with the subprocess library. (Find out here)

I also found an answer of another question that uses check_ouput() in order to know when the python program has finished. This can also be useful if you want to send a status to the tkinter app: you can print("33% Complete"), for example. You could add this in tkinter's main loop, so you always know if your program is running or not.

And last but not least, to kill that process (using the stop button), you should do it using os, and looking for the subprocess' ID. Here you can also find a good example.

I would try something like this:

cmd = "exec python file.py"
p = subprocess.Popen(cmd, shell=True)
# Continue running tkinter tasks.
tk.update()
tk.update_idletasks() # These both lines should be inside a while True
# Stop secondary program
p.kill()

EDIT

Example code using your question's code. WARNING: I have changed the png file location for testing, commented the app icon, and tested ONLY on Windows.

It's important to remove the mainloop() on the main file and put update...() in order to catch the keyboardInterrupt that (I don't know why) is killing both parent and child process.

I invite you to try it and be as happy as I have been when it was working after half an hour of testing!!

File 1: daemon.py - this file will run forever.

from time import sleep
from sys import exit

while True:
    try:
        print("hello")
        sleep(1)
    except KeyboardInterrupt:
        print("bye")
        exit()

File 2: tkinterapp.py - The name is self-explainatory

import tkinter as tk
import subprocess
import sys
import time
import os
import tkinter.font as font
from tkinter.ttk import *

app = tk.Tk()
app.geometry("400x400")
app.configure(bg='gray')

photo = tk.PhotoImage(file=r"C:\Users\royal\github\RandomSketches\baixa.png")
myFont = font.Font(family='Helvetica', size=20, weight='normal')

tk.Label(app, text='EX', bg='gray', font=(
    'Verdana', 15)).pack(side=tk.TOP, pady=10)
# app.iconbitmap(r'C:\Users\ex\ex_icon.ico')


def ex_activation():
    global pro
    print("running!")
    pro = subprocess.Popen("python daemon.py", shell=True)

def ex_stop():
    global pro
    print("stopping!")
    os.kill(pro.pid, 0)

ex_activation_button = tk.Button(app,
                                    bg='black',
                                    image=photo,
                                    width=120,
                                    height=120,
                                    command=ex_activation)
ex_stop_button = tk.Button(app,
                              bg='Gray',
                              text='ex',
                              width=12,
                              command=ex_stop, # BE CAREFUL You were missing a "," here !!!
                              height=3)
ex_stop_button['font'] = myFont

app.title("Example")
ex_activation_button.pack(side=tk.TOP)
ex_stop_button.pack(side=tk.LEFT)

# app.mainloop()
while True:
    try:
        app.update()
        app.update_idletasks()
    except KeyboardInterrupt:
        pass