[PyQt] Threads, events, signals and slots...

Peter Milliken peter.milliken at gmail.com
Wed Aug 25 01:51:37 BST 2010

Slowly working my way to learning this stuff but have a question regarding
how to update GUI elements when threads are running.

The situation is:

1. I have some radiobuttons that I use for input and output: input by the
user and then for "output" to display "progress" during an operation (the
"operation" is a task that runs in a QThread instance). The radiobuttons are
grouped into a QGroupBox are there are 3 of them to indicate 3 phases of a
total operation. The user selects a start phase and then instigates a
process (via a QCommandLinkButton), as the process progresses from the start
phase to the final phase the phase buttons must be updated.

2. I have a QProgressBar to indicate the progress (0 - 100%) within each
phase (indicated by the previously mentioned phase buttons). So each phase
progresses through 0 - 100% completion before advancing to the next phase.

So basically the user selects which phase they want to start at (0, 1 or 2)
and then press a button to instigate a QThread that performs the actions.
The progress is communicated back from the thread via messages in a pipe. To
receive the messages and update the GUI elements I start another QThread -
this is one of the methods of the application GUI class and goes into a
while loop, receiving messages from the thread that is doing the job and
directly making calls to GUI elements like:

self.phase_0.setChecked(True) and

At the end of the while loop I do a time.sleep() command to allow the Qt
event loop to run and presumably update any GUI elements that need
"updating". So the code looks like this (mix of pseudo code and real code

while True:
  if pipe.poll():
    msg = pipe.rcv()
    if msg is phase 0:
    elif msg = progress:
    elif msg = stop

This seems to work quite well, however, every now and again, when switching
between Windows applications etc while the process that I have tasked is
running, I get an error/warning message:

"QWidget::repaint: Recursive repaint detected"

Should I be using signals/slots to update the GUI elements from within the
"2nd" task? If so, what would it look like (please use an example that would
see one of the "phase" radiobuttons updated).

Thanks for reading this and providing any help you can :-)

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