Tkinter wait_variable problem: hangs at termination

Russell E. Owen rowen at cesmail.net
Tue Jun 8 20:41:20 CEST 2004


I want to support execution of simple user-written scripts in a Tkinter 
application. The scripts should be able to wait for data and such 
without hanging the GUI (and without having to write the script as a 
bunch of asynchronously called subroutines).

I decided to use Tkinter's wait_variable. I built a "script runner" 
object that has suitable wait methods. Internally each of these methods 
registers a callback that sets a variable when the wait condition is 
satisfied, then calls wait_variable to wait until the variable is set.

The resulting scripts are simple and seem to work well, e.g.:

myscript(sr):
    # do some normal python stuff that is fast
    sr.waitForData(...)
    # more normal fast python stuff
    sr.waitForMS(time)
    # etc.

Unfortunately, if a user closes the root window while wait_variable is 
waiting, the application never fully quits. On my unix box the root 
window closes but the command-line prompt never returns and ^C is 
ignored. The process isn't using excess cpu cycles; it's just not 
listening.

I have tried registering an atexit handler and adding a __del__ method 
so that the variable being waited on is toggled at shutdown, but it 
makes no difference.

Here is an example:
<ftp://ftp.astro.washington.edu/pub/users/rowen/python/WaitBug.py>
Press "Start" to start the script (which simply prints a number to 
sys.stdout every second, then quits. Close the root window while the 
script is waiting to print the next number, or after pausing the script, 
and you'll see the problem.

Any suggestions for how to avoid/work around this problem? If it's a 
Tkinter bug I'll report it.

-- Russell

P.S. I saw that some perl/Tk users got around a different problem by 
faking wait_variable by running a tight "do one event, see if the wait 
condition is satisfied" loop.

I don't think Tkinter allows one to execute a single event, and I 
suspect it would be inefficient even if it were possible. I might be 
able to use update (though the nice thing about wait_variable is most of 
the action happens down at the tcl/tk level, presumably making it 
maximally efficient).

(This is one of those rare times I wish Tkinter worked at the C level 
the way perl's tk interface does.)



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