How to solve the problem with threads?

Question

Python

How to fix the error?
The first message gives, the next gives an error

from threading import Thread
from tkinter import messagebox

header_msg = "5555"

def fn(mess_msg, header_msg):
  messagebox.showerror(str(header_msg), str(mess_msg))

while True:
  mess_msg = input(">>>: ")
  th = Thread(target=fn, args=(mess_msg, header_msg))
  th.start()

Mistake

$ python r.py
>>>: afsfs
>>>: dasd
>>>: Exception in thread Thread-2:
Traceback (most recent call last):
  File "C:\Users\User\AppData\Local\Programs\Python\Python38\lib\threading.py", line 932, in _bootstrap_inner
    self.run()
  File "C:\Users\User\AppData\Local\Programs\Python\Python38\lib\threading.py", line 870, in run
    self._target(*self._args, **self._kwargs)
  File "r.py", line 14, in fn
    messagebox.showerror(str(header_msg), str(mess_msg))
  File "C:\Users\User\AppData\Local\Programs\Python\Python38\lib\tkinter\messagebox.py", line 94, in showerror
    return _show(title, message, ERROR, OK, **options)
  File "C:\Users\User\AppData\Local\Programs\Python\Python38\lib\tkinter\messagebox.py", line 72, in _show
    res = Message(**options).show()
  File "C:\Users\User\AppData\Local\Programs\Python\Python38\lib\tkinter\commondialog.py", line 40, in show
    w = Frame(self.master)
  File "C:\Users\User\AppData\Local\Programs\Python\Python38\lib\tkinter\__init__.py", line 3119, in __init__
    Widget.__init__(self, master, 'frame', cnf, {}, extra)
  File "C:\Users\User\AppData\Local\Programs\Python\Python38\lib\tkinter\__init__.py", line 2567, in __init__
    self.tk.call(
RuntimeError: main thread is not in main loop

Answer 1

Tk, like any other GUI library, is not thread-safe, so GUI components can only be accessed from the main thread.