Strange threading behaviour

[Rotwang]

On 21/06/2012 18:37, Dennis Lee Bieber wrote:
> On Thu, 21 Jun 2012 16:19:41 +0100, Rotwang<sg552 at hotmail.co.uk>
> declaimed the following in gmane.comp.python.general:
>
>>
>> import threading, Tkinter, os, pickle
>>
>> class savethread(threading.Thread):
>>       def __init__(self, value):
>>           threading.Thread.__init__(self)
>>           self.value = value
>>       def run(self):
>>           print 'Saving:',
>>           with open(os.path.join(os.getcwd(), 'bugfile'), 'wb') as f:
>>               pickle.dump(self.value, f)
>>           print 'saved'
>>
>> class myclass(object):
>>       def gui(self):
>>           root = Tkinter.Tk()
>>           root.grid()
>>           def save(event):
>>               savethread(self).start()
>>           root.bind('s', save)
>
> 	Each time your "s" runs you are creating a NEW thread.

Yes, that's the whole point: I want the object's state to be saved in 
the background while I can continue using the GUI without having to wait 
(in the original module where I noticed the problem, the thread pickles 
a copy of the object whose gui method created it, rather than the object 
itself - that way I can make changes to the object without interfering 
with the work being done by pickle.dump).


>
>>           root.wait_window()
>>
>> m = myclass()
>> m.gui()
>>
>> --- end bugtest.py ---
>>
>>
>> Here's the problem: suppose I fire up Python and type
>>
>>   >>>  import bugtest
>>
> 	Running in the interpreter rather than as a program...
>
>> and then click on the Tk window that spawns and press 's'. Then
>> 'Saving:' gets printed, and an empty file named 'bugfile' appears in my
>> current working directory. But nothing else happens until I close the Tk
>> window; as soon as I do so the file is written to and 'saved' gets
>> printed. If I subsequently type
>>
>>   >>>  bugtest.m.gui()
>>
>> and then click on the resulting window and press 's', then 'Saving:
>> saved' gets printed and the file is written to immediately, exactly as I
>> would expect. Similarly if I remove the call to m.gui from the module
>> and just call it myself after importing then it all works fine. But it
>> seems as if calling the gui within the module itself somehow stops
>> savethread(self).run from finishing its job while the gui is still alive.
>>
> 	Since you never kill the previous thread, you might have a binding
> to left-over multiple threads; at the least, the Python scheduler may be
> swapping between threads.

Which previous thread do you mean? The problem happens when save has 
only been called once.


> 	The task swapping probably gives the threads time to flush output.
> With just the first run thread condition, the process may go back to the
> Tk mainloop and not return to the thread to completely flush data.
>
> 	Especially with the use of .wait_window(), which is supposed to lock
> the application until the window is destroyed. If overly aggressive, it
> might even lock up swapping back to the thread to allow it to flush.
>
>> Can anyone help?
>
> 	Don't run from the interactive interpreter?

Thanks.

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