Why does the update method in Tkinter cause the window to freeze?

Question

First of all, I know Tkinter isn't thread safe and this problem has something to do with that but I wanted to find out formally why this code makes a window that displays but is unresponsive.

from Tkinter import *
root = Tk()
c = Canvas()
c.pack()
c.create_line(10,10, 30, 30)
root.update()

I want to know why it crashes. I know the last line should contain a mainloop() but if as this post says mainloop just continuously calls the two methods there is no reason the above code should be unresponsive.

Do you really mean that the program crashes, or is it simply exiting? — Bryan Oakley, Jun 22 '15 at 16:29
Yes. Try it. It just stops responding and you have to force quit the window. — Michael Chav, Jun 22 '15 at 18:46
What platform? I don't see how it is possible for it to become unresponsive if called in the main thread of the app, with no other threads. Once `root.update()` returns, there's nothing left for the program to do but exit, and there isn't anything preventing it from returning. — Bryan Oakley, Jun 22 '15 at 18:48
Considering the _exact_ code won't even run, I think there must be more details that you've left out. — Bryan Oakley, Jun 22 '15 at 18:52
I've run your code both on OSX and Windows, and in both cases it exits rather than hangs, as expected. Are you running on linux? — Bryan Oakley, Jun 22 '15 at 19:46

fferri · Accepted Answer · 2015-06-22T17:11:31.067

3

update() just processes events once.

mainloop() (as the name implies) is a loop that continuously processes events.

In your code you call root.update() only once, that's why your program becomes unresponsive or terminates.

Put the call to root.update() in a while loop, and it will work as expected:

while True:
    root.update()

However, if this solves your problem, probably you just want to call root.mainloop().

edited Jun 22 '15 at 17:11

answered Jun 22 '15 at 15:27

fferri

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Why do you advocate putting `update` in a loop rather than calling `mainloop`? Your solution is a bit less efficient than calling `mainloop`. – Bryan Oakley Jun 22 '15 at 16:27
The OP premise is to **not** use `mainloop()`. I have no bias toward one or another solution. – fferri Jun 22 '15 at 17:10

Bryan Oakley · Answer 2 · 2015-09-08T12:41:14.507

The reason it exits (not crashes) is because it has done everything you've asked it to do. You told it to process any pending events, and then like every other python program, when it runs out of statements, it exits.

A GUI needs to be able to continuously process events. In tkinter you do that by calling the mainloop method of the root window. This will not return (and thus, your program won't exit) until the root window has been destroyed.

from Tkinter import *
root = Tk()
...
root.mainloop()

If your real code is running in a thread, the problem is likely due to threading. The exact code in your question, when run in a single threaded interpreter, will exit almost immediately after the root window is displayed.

It doesn't exit. It just freezes and stops responding to input. I think that's a crash, right? — Michael Chav, Jun 22 '15 at 18:31

Why does the update method in Tkinter cause the window to freeze?

2 Answers2