Closing my tkinter serial app, gives me an exception

Question

I have this program that grabs data from serial and displays them on a tkinter frame.

This is the code:

import tkinter as tk
import tkinter.ttk as ttk
import serial.tools.list_ports
from tkinter import scrolledtext 
#new stuff from vid
import time
import serial
import threading
import continuous_threading


#to be used on our canvas
HEIGHT = 700
WIDTH = 800

#hardcoded baud rate
baudRate = 9600



ser = serial.Serial('COM16', baudRate)
val1 = 0
index = []
def readSerial():
    global val1
    ser_bytes = ser.readline()
    ser_bytes = ser_bytes.decode("utf-8")
    val1 = ser_bytes
    scrollbar.insert("end", val1)
    
t1 = continuous_threading.PeriodicThread(0.1, readSerial)
#----------------------------------------------------------------------

# --- functions ---

#the following two functtions are for the seria port selection, on frame 1
def serial_ports():    
    return serial.tools.list_ports.comports()

def on_select(event=None):

    global COMPort
    COMPort = cb.get()
    print(COMPort)



# --- functions ---


#--------------------------------------------------------------------------------------------------------------
# --- main ---
root = tk.Tk() #here we create our tkinter window
root.title("Sensor Interface")

#we use canvas as a placeholder, to get our initial screen size (we have defined HEIGHT and WIDTH)
canvas = tk.Canvas(root, height=HEIGHT, width=WIDTH)
canvas.pack()

#we use frames to organize all the widgets in the screen
# --- frame 1 ---
frame1 = tk.Frame(root)
frame1.place(relx=0, rely=0.05, relheight=0.03, relwidth=1, anchor='nw') #we use relheight and relwidth to fill whatever the parent is - in this case- root

label0 = tk.Label(frame1, text="Select the COM port that the device is plugged in: ")
label0.config(font=("TkDefaultFont", 8))
label0.place(relx = 0.1, rely=0.3, relwidth=0.3, relheight=0.5)


cb = ttk.Combobox(frame1, values=serial_ports())
cb.place(relx=0.5, rely=0.5, anchor='center')
# assign function to cmbobox
cb.bind('<<ComboboxSelected>>', on_select)
# --- frame 1 ---




# --- frame 2 ---
frame2 = tk.Frame(root, bg='#80c1ff') #remove color later
frame2.place(relx=0, rely=0.1, relheight=1, relwidth=1, anchor='nw')

# make a scrollbar
scrollbar = scrolledtext.ScrolledText(frame2)
scrollbar.place(relx=0, rely=0, relheight=1, relwidth=1, anchor='nw')
# --- frame 2 ---
#--------------------------------------------------------------------------------------------------------
t1.start() 

root.mainloop() #here we run our app

When i terminate the GUI that spawns, i get this exception on the terminal:

Exception in thread Thread-1:
Fatal Python error: could not acquire lock for <_io.BufferedWriter name='<stderr
>'> at interpreter shutdown, possibly due to daemon threads
Python runtime state: finalizing (tstate=005C8F38)

Thread 0x00001a30 (most recent call first):
  File "C:\Users\User1\AppData\Local\Programs\Python\Python38-32\lib\threading.p
y", line 1202 in invoke_excepthook
  File "C:\Users\User1\AppData\Local\Programs\Python\Python38-32\lib\threading.p
y", line 934 in _bootstrap_inner
  File "C:\Users\User1\AppData\Local\Programs\Python\Python38-32\lib\threading.p
y", line 890 in _bootstrap

Current thread 0x00001918 (most recent call first):
<no Python frame>

And when i terminate it from the terminal with control+C i get:

Traceback (most recent call last):
  File "mySerial.py", line 110, in <module>
    root.mainloop() #here we run our app
  File "C:\Users\User1\AppData\Local\Programs\Python\Python38-32\lib\tkinter\__i
nit__.py", line 1420, in mainloop
    self.tk.mainloop(n)
KeyboardInterrupt
Exception in thread Thread-1:
Fatal Python error: could not acquire lock for <_io.BufferedWriter name='<stderr
>'> at interpreter shutdown, possibly due to daemon threads
Python runtime state: finalizing (tstate=00578F38)

Thread 0x00001ad0 (most recent call first):
  File "C:\Users\User1\AppData\Local\Programs\Python\Python38-32\lib\threading.p
y", line 1202 in invoke_excepthook
  File "C:\Users\User1\AppData\Local\Programs\Python\Python38-32\lib\threading.p
y", line 934 in _bootstrap_inner
  File "C:\Users\User1\AppData\Local\Programs\Python\Python38-32\lib\threading.p
y", line 890 in _bootstrap

Current thread 0x000013d8 (most recent call first):
<no Python frame>

Why is this hapenning?

EDIT: This is my code and error message, trying AST's advice:

import tkinter as tk
import tkinter.ttk as ttk
import serial.tools.list_ports
from tkinter import scrolledtext 
import time
import serial
import threading
import continuous_threading


#to be used on our canvas
HEIGHT = 700
WIDTH = 800

#hardcoded baud rate
baudRate = 9600

# flag to be notified when application is terminated
stop=False


ser = serial.Serial('COM16', baudRate)
val1 = 0

def readSerial():
    global val1, stop
    if not stop:
       ser_bytes = ser.readline()
       ser_bytes = ser_bytes.decode("utf-8")
       val1 = ser_bytes
       scrollbar.insert("end", val1)
    else:
        return

t1 = continuous_threading.PeriodicThread(0.1, readSerial)
#----------------------------------------------------------------------

# --- functions ---

#the following two functions are for the seria port selection, on frame 1
def serial_ports():    
    return serial.tools.list_ports.comports()

def on_select(event=None):

    global COMPort
    COMPort = cb.get()
    print(COMPort)

def on_close():
    global stop
    stop=True
    root.destroy()



# --- functions ---


#--------------------------------------------------------------------------------
# --- main ---
root = tk.Tk() #here we create our tkinter window
root.title("Sensor Interface")
root.protocol('WM_DELETE_WINDOW', on_close)

#we use canvas as a placeholder, to get our initial screen size (we have defined HEIGHT and WIDTH)
canvas = tk.Canvas(root, height=HEIGHT, width=WIDTH)
canvas.pack()

#we use frames to organize all the widgets in the screen
# --- frame 1 ---
frame1 = tk.Frame(root)
frame1.place(relx=0, rely=0.05, relheight=0.03, relwidth=1, anchor='nw') #we use relheight and relwidth to fill whatever the parent is - in this case- root

label0 = tk.Label(frame1, text="Select the COM port that the device is plugged in: ")
label0.config(font=("TkDefaultFont", 8))
label0.place(relx = 0.1, rely=0.3, relwidth=0.3, relheight=0.5)


cb = ttk.Combobox(frame1, values=serial_ports())
cb.place(relx=0.5, rely=0.5, anchor='center')
# assign function to cmbobox
cb.bind('<<ComboboxSelected>>', on_select)
# --- frame 1 ---




# --- frame 2 ---
frame2 = tk.Frame(root, bg='#80c1ff') #remove color later
frame2.place(relx=0, rely=0.1, relheight=1, relwidth=1, anchor='nw')

# make a scrollbar
scrollbar = scrolledtext.ScrolledText(frame2)
scrollbar.place(relx=0, rely=0, relheight=1, relwidth=1, anchor='nw')
# --- frame 2 ---
#--------------------------------------------------------------------------------
t1.start() 
root.mainloop() #here we run our app

The error i get, is the first error i have listed as code.

EDIT 2: When i do the second approach of AST:

If i close the GUI (pressing X), it closes without any errors, but the prompt is stuck on the terminal - i cannot type anything, not even keyboard exit (Control+C) works.

If the program runs and i exit with the keyboard exit in the terminal (Control+C), i get the keyboard interrupt error (the second one i have listed as code)

Answer 1

According to me, this occurs because there could be a scheduled thread that gets executed after the application has been destroyed and hence fails to find the GUI element that it has to update.

I haven't tried running your code, but I think this might help

Associate the window deletion with a function that updates the flag (stop)

def on_close():
    global stop
    stop=True
    root.destroy()

root.protocol('WM_DELETE_WINDOW', on_close)
stop=False

And modify the readSerial function to check for the same

def readSerial():
    global val1,stop
    if not stop:
        ser_bytes = ser.readline()
        ser_bytes = ser_bytes.decode("utf-8")
        val1 = ser_bytes
        scrollbar.insert("end", val1)
    else:
        return

EDIT

This is the function in threading.py that raises the exception.

def _bootstrap(self):
    # Wrapper around the real bootstrap code that ignores
    # exceptions during interpreter cleanup.  Those typically
    # happen when a daemon thread wakes up at an unfortunate
    # moment, finds the world around it destroyed, and raises some
    # random exception *** while trying to report the exception in
    # _bootstrap_inner() below ***.  Those random exceptions
    # don't help anybody, and they confuse users, so we suppress
    # them.  We suppress them only when it appears that the world
    # indeed has already been destroyed, so that exceptions in
    # _bootstrap_inner() during normal business hours are properly
    # reported.  Also, we only suppress them for daemonic threads;
    # if a non-daemonic encounters this, something else is wrong.
    try:
        self._bootstrap_inner()
    except:
        if self._daemonic and _sys is None:
            return
        raise

As in this case it's okay to kill the thread when all the non-daemonic threads have been killed, you can try setting the thread as daemonic so that the exception is automatically handled. I don't think the flag would be required with this approach.

t1.daemon=True
t1.start()