Runtimeerror: Main Thread Is Not in Main Loop

You're running your main GUI loop in a thread besides the main thread. You cannot do this.

The docs mention offhandedly in a few places that Tkinter is not quite thread safe, but as far as I know, never quite come out and say that you can only talk to Tk from the main thread. The reason is that the truth is somewhat complicated. Tkinter itself is thread-safe, but it's hard to use in a multithreaded way. The closest to official documentation on this seems to be this page:

Q. Is there an alternative to Tkinter that is thread safe?

Tkinter?

Just run all UI code in the main thread, and let the writers write to a Queue object…

(The sample code given isn't great, but it's enough to figure out what they're suggesting and do things properly.)

There actually is a thread-safe alternative to Tkinter, mtTkinter. And its docs actually explain the situation pretty well:

Although Tkinter is technically thread-safe (assuming Tk is built with --enable-threads), practically speaking there are still problems when used in multithreaded Python applications. The problems stem from the fact that the _tkinter module attempts to gain control of the main thread via a polling technique when processing calls from other threads.

I believe this is exactly what you're seeing: your Tkinter code in Thread-1 is trying to peek into the main thread to find the main loop, and it's not there.

So, here are some options:

• Do what the Tkinter docs recommend and use TkInter from the main thread. Possibly by moving your current main thread code into a worker thread.
• If you're using some other library that wants to take over the main thread (e.g., twisted), it may have a way to integrate with Tkinter, in which case you should use that.
• Use mkTkinter to solve the problem.

Also, while I didn't find any exact duplicates of this question, there are a number of related questions on SO. See this question, this answer, and many more for more information.

I know this is late, but I set my thread to a Daemon, and no exception was raised:

t = threading.Thread(target=your_func)
t.setDaemon(True)
t.start()

I found a way to solve it. it might look like a joke but you just should add

plt.switch_backend('agg')

Since all this did help my problem but did not solve it completely here is an additional thing to keep in mind:

In my case I started off importing the pyplot library in many threads and using it there. After moving all the library calls to my main thread I still got that error.

I did get rid of it by removing all import statements of that library in other files used in other threads. Even if they did not use the library the same error was caused by it.

from tkinter import *
from threading import Thread
from time import sleep
from random import randint

class GUI():

    def __init__(self):
        self.root = Tk()
        self.root.geometry("200x200")

        self.btn = Button(self.root,text="lauch")
        self.btn.pack(expand=True)

        self.btn.config(command=self.action)

    def run(self):
        self.root.mainloop()

    def add(self,string,buffer):
        while  self.txt:
            msg = str(randint(1,100))+string+"\n"
            self.txt.insert(END,msg)
            sleep(0.5)

    def reset_lbl(self):
        self.txt = None
        self.second.destroy()

    def action(self):
        self.second = Toplevel()
        self.second.geometry("100x100")
        self.txt = Text(self.second)
        self.txt.pack(expand=True,fill="both")

        self.t = Thread(target=self.add,args=("new",None))
        self.t.setDaemon(True)
        self.t.start()

        self.second.protocol("WM_DELETE_WINDOW",self.reset_lbl)

a = GUI()
a.run()

maybe this example would help someone.

Write it at the end:

root.mainloop()

Of course, in place of root should be the name of your Tk object if it is not root.

You cannot modify your main GIU from another thread you need to send event to the main GUI in order to avoid exceptions Use window.write_event_value instead, this method allows you to send events from your threads you can take a look a this too: window.perform_long_operation