Intermittent Python thread error, “main thread is not in main loop”

Question

Middle aged dad (electrical engineer not programmer by trade) trying to teach my 13 year old daughter electronics and programming. So far, I love Python. I am building a pro

Answer 1

You need to run the GUI code in the main thread, and your temperature reading code needs to be in the background thread. It's only safe to update the GUI in the main thread, so you can pass the temperature data you're reading from the background thread back to the main thread via a Queue, and have the main thread periodically check for data in the queue using self.root.after():

from Tkinter import *
import tkFont
import os
import glob
import time
import threading
import Image 
import Queue


def update_temp(queue):
    """ Read the temp data. This runs in a background thread. """
    while True:
        #   28-000005c6ba08
        i = "28-000005c6ba08"
        base_dir = '/sys/bus/w1/devices/'
        device_folder = glob.glob(base_dir + i)[0]
        device_file = device_folder + '/w1_slave'

        tempread=round(read_temp(),1)

        # Pass the temp back to the main thread.
        queue.put(tempread)
        time.sleep(5)

class Gui(object):
    def __init__(self, queue):
        self.queue = queue

        #Make the window
        self.root = Tk() 
        self.root.wm_title("Home Management System")
        self.root.minsize(1440,1000)

        self.equipTemp = StringVar()   
        self.equipTemp1 = StringVar()
        self.equipTemp2 = StringVar()       

        self.customFont = tkFont.Font(family="Helvetica", size=16)

        #   1st floor Image
        img = Image.open("HOUSE-PLANS-01.png") 
        photo = ImageTk.PhotoImage(img)

        Label1=Label(self.root, image=photo)
        Label1.place(x=100, y=100)

        #   2nd floor
        img2 = Image.open("HOUSE-PLANS-02.png")
        photo2 = ImageTk.PhotoImage(img2)

        Label1=Label(self.root, image=photo2)
        Label1.place(x=600, y=100)

        #   Basement image
        img3 = Image.open("HOUSE-PLANS-03.png")
        photo3 = ImageTk.PhotoImage(img3)

        Label1=Label(self.root, image=photo3)
        Label1.place(x=100, y=500)

        #   Attic Image
        img4 = Image.open("HOUSE-PLANS-04.png")
        photo4 = ImageTk.PhotoImage(img4)

        Label1=Label(self.root, image=photo4)
        Label1.place(x=600, y=500)

        #   House Isometric Image
        img5 = Image.open("house-iso.png")
        photo5 = ImageTk.PhotoImage(img5)

        Label1=Label(self.root, image=photo5)
        Label1.place(x=1080, y=130)

        #Garage Temp Label
        Label2=Label(self.root, textvariable=self.equipTemp, width=6, justify=RIGHT, font=self.customFont)
        Label2.place(x=315, y=265)

        print "start monitoring and updating the GUI"

        # Schedule read_queue to run in the main thread in one second.
        self.root.after(1000, self.read_queue)

    def read_queue(self):
        """ Check for updated temp data"""
        try:
            temp = self.queue.get_nowait()
            self.equipTemp.set(temp)
        except Queue.Empty:
            # It's ok if there's no data to read.
            # We'll just check again later.
            pass
        # Schedule read_queue again in one second.
        self.root.after(1000, self.read_queue)

if __name__ == "__main__":
    queue = Queue.Queue()
    # Start background thread to get temp data
    t = threading.Thread(target=update_temp, args=(queue,))
    t.start()
    print "starting app"
    # Build GUI object
    gui = Gui(queue)
    # Start mainloop
    gui.root.mainloop()

Edit:

After actually taking a look at the tkinter source code, as well as the Python bug tracker, it appears that unlike almost every other GUI library out there, tkinter is intended to be thread-safe, as long you run the mainloop in the main thread of the application. See the answer I added here for more info, or go straight to the resolved issue about tkinter's thread safety on the Python bug tracker here. If the tkinter source and Python's bug tracker are correct, that would mean that as long as you run the mainloop in the main thread, you can happily call gui.equipTemp.set() directly from your temperature reading thread - no Queue required. And in my testing, that did indeed work just fine.