RuntimeError: main thread is not in main loop

by

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.

More Related Contents:

  1. Tkinter: How to use threads to preventing main event loop from “freezing”
  2. Freezing/Hanging tkinter GUI in waiting for the thread to complete
  3. Threads and tkinter
  4. Program freezing during the execution of a function in Tkinter
  5. Mutli-threading python with Tkinter
  6. Tkinter GUI, I/O & Threading: When to use queues, when events?
  7. Running a Tkinter form in a separate thread
  8. Trying to fix tkinter GUI freeze-ups (using threads)
  9. Threaded Tkinter script crashes when creating the second Toplevel widget
  10. How to start threads at the same time in Python [duplicate]
  11. Python Tkinter Label redrawing every 10 seconds
  12. How to limit execution time of a function call?
  13. How to get variable data from a class?
  14. Python threading multiple bash subprocesses?
  15. How to update the image of a Tkinter Label widget?
  16. Unable to load files using pickle and multiple modules
  17. The right way to limit maximum number of threads running at once?
  18. How would I make a method which is run every time a frame is shown in tkinter
  19. How to create equal-width grid columns with Tkinter?
  20. Add advanced features to a tkinter Text widget
  21. Why is my Button’s command executed immediately when I create the Button, and not when I click it? [duplicate]
  22. How to connect a progress bar to a function?
  23. Python thread name doesn’t show up on ps or htop
  24. Make Anaconda’s tkinter aware of system fonts or install new fonts for Anaconda
  25. Threads are printing at the same time messing up the text output
  26. Access a QLCDNumber object from a external function
  27. stop tkinter window from freezing while program is sleeping
  28. I am getting { } on the end of my output. What is causing this and how do I remove it?
  29. Using multiprocessing.Process with a maximum number of simultaneous processes
  30. ModX in event.state in tkinter?

Leave a Comment