[PyKDE] Threading QApplications. Possible?

[David Boddie]

On Mon, 20 Sep 2004 20:52:01, Trost Johnson wrote: 
 
> Suppose the minimal program: 
> 
> def foo(): 
>     app=QApplication(sys.argv) 
>     button=QPushButton("Hello World", None) 
>     app.setMainWidget(button) 
>     button.show() 
>     app.exec_loop() 
> 
> And I say 
> 
> >>> thread.start_new_thread (foo, ()) 
> 
> Then the python console freezes.... 
 
I'm not completely surprised by this. :-) 
 
> Is it not possible o thread up multiple QApplications? 
 
I don't think you can create multiple QApplications in this way. Not within 
the same application, anyway. 
 
> How is this achieved? 
 
I think that you really need QApplication's event loop to be running in 
the main Python thread. This prevents you from running it in a separate 
thread, but you can still run other tasks in separate Python threads. 
Just be careful when communicating back to the QApplication from these 
other threads. 
 
Note that there is a separate problem with running the event loop from 
within an interactive Python session: the interpreter doesn't regain 
control until after the QApplication finishes. I presume that you are 
trying to run the event loop in a separate thread to solve this problem. 
 
I found that it is quite useful to consider approaching the problem from 
another angle and implement parts of an interactive prompt within a 
Qt-based application. Although the behaviour of the interpreter isn't 
quite what you'd expect from a normal interactive Python session, you can 
get quite far with this approach. 
 
Take a look at qpython from the following page: 
 
  http://www.boddie.org.uk/david/Projects/Python/KDE/ 
 
The code isn't very nice to look at, and I think that there's a race 
condition involving the readline module, but it lets you use Qt-based 
modules from within an interactive Python session. 
 
Good luck! 
 
David 


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