[PyKDE] QButton/QGridLayout woes

Hoka "ME" Tichenci melkin at uci.edu
Tue Sep 20 19:07:22 BST 2005

What I mean from a multithreaded application is that the threaded object 
makes calls into my QT class, and the QT class modifies the widgets 
accordingly (the qt app is single threaded). The given code was within the 
QT application. Originally this didn't work because the PyQT I was using 
was based on SIP3 (which caused lockups in this exact scenario), but I 
should now be using a SIP4 release (3.14.1 edu). I'm also using a locking 
mechanism so that this code should run completely before it can be run 
again (so it should be threadsafe). I think its odd because the message box 
will pop up if I construct it, but trying to add the button doesn't work.


>You mention "from a multithreaded application". Are you being careful
>to keep all Qt activity exclusively in one thread? From http:// 
>In Qt, one thread is always the GUI or event thread. This is the
>thread that creates a QApplication object and calls QApplication::exec (). 
>This is also the initial thread that calls main() at program
>start. This thread is the only thread that is allowed to perform GUI
>operations, including generating and receiving events from the window
>system. Qt does not support creating QApplication and running the
>event loop (with QApplication::exec()) in a secondary thread. You
>must create the QApplication object and call QApplication::exec()
>from the main() function in your program.
>Threads that wish to display data in a widget cannot modify the
>widget directly, so they must post an event to the widget using
>QApplication::postEvent(). The event will be delivered later on by
>the GUI thread.
>My guess would be that two threads got into Qt at the same time. It
>may not happen under Linux due to timing differences.
>On Sep 18, 2005, at 10:07 PM, Hoka ME Tichenci wrote:
>>I'm having an odd issue with Windows PyQt 3.14.1 Educational, the
>>code is supposed to dynamically add/remove buttons when called from
>>a multithreaded application. On Linux (PyQt 3.13 qt 3.3.4) the code
>>runs fine, but on Windows I get assert problems in a rather odd spot.
>>The code is here (bgrid is a QGridLayout, bg_projectors is a
>>print "adding projector rnode: %s" % rnode.getName()
>>attrs = rnode.getAttributes()
>>print "xpos: %s ypos: %s" % (attrs["xpos"],attrs["ypos"])
>>tmp = QPushButton(self.bg_projectors,"yes")
>>print "qpushbutton built"
>>print "widget added"
>>print "Done adding projector!"
>>However, when run I get this output:
>>adding projector rnode: Projector: BigOne
>>xpos: 1 ypos: 1
>>qpushbutton built
>>projector is off
>>widget added
>>Done adding projector!
>>ASSERT: "src_dc && dst_dc" in kernel\qpaintdevice_win.cpp (388)
>>What's odd is that the ASSERT failure is at the end, after the
>>method should be done and printed its last statement. Furthermore,
>>if I make a QWarningBox or other similar piece of code pauses the
>>system after tmp.show() is called, the QButton can be seen in the
>>background, after the box is closed the button disappears, but
>>oddly enough no ASSERT is thrown. This may not be the right way to
>>do something like this, and if it isn't correct me, but I'm at a
>>lost and Google didn't return anything worthwhile.
>>Thanks for your time,

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