Realtime TextItem update

[Toni]

Hi everyone,

I'm trying to update the text inside a TextItem object from a callback, so that I can stream float values that come from outside my python script.

The problem I'm facing is that once my callback is invoked from the framework I'm using, the following error is thrown:

QObject: Cannot create children for a parent that is in a different thread.

I don't know how to avoid that and I'm not sure if there is actually a way to do what I want,

This is the code I have written until now:

QtGui.QApplication([])

        

## Create window with GraphicsView widget

        

win = pg.GraphicsLayoutWidget()

win.show()  ## show widget alone in its own window        

win.setWindowTitle('...')

        

# ... window settings

        

## Create image item

self.vis = pg.ImageItem()

        

# ... image settings        

       

## Create obstacle histogram  

plot = pg.PlotItem()

       

# ... plot settings

self.text = pg.TextItem('Loading...')

self.view.addItem(self.text)

And from the callback mentioned above (where the problem is) I do:

self.text.setText('Hello world!')

Does anyone know how to avoid this problem and implement this behavior?

Thank you in advance for your answers, I hope the question is clear.

 

       

[Sebastian Höfer]

Hello Toni,

judging from the error message the problem ist not in the code you postet, but in the way you use the callback. Like the error message states: you shouldn't call methods of QObjects from other threads. Even if you don't use threads explicitly that happens easily if you use another framework/module that makes use of threads internally. So your callback is probably invoked in another thread.

You can work around that by using signals. 

Define your own signal...  for example 

my_signal = QtCore.pyqtSignal()

and in your callback function you trigger the signal 

my_signal.emit()

In your GUI code you connect your new signal with the update function 

my_signal.connect(lambda: self.text.setText('Hello world!'))

If the concept is new to you, here's a tutorial:  http://pythoncentral.io/pysidepyqt-tutorial-creating-your-own-signals-and-slots/

Regards

Sebastian

[Toni]

Hello Sebastian,

thank you very much for your fast and useful answer, everything works well now with signals, and the tutorial you've linked above help me understand what I've done.

I'll post right here the fixed version of my code in case someone has the same problem.

class Test(QtCore.QObject):

    

    my_signal = QtCore.pyqtSignal()

    

    def __init__(self):

        QtCore.QObject.__init__(self)

        

        # ... stuffs

        

        app = QtGui.QApplication([])

        

        ## Create window with GraphicsView widget

        win = pg.GraphicsLayoutWidget()

       
        # ... window settings

        

        ## Create image item

        self.vis = pg.ImageItem()
        
        # ... image settings

        

        ## Create obstacle histogram

        plot = pg.PlotItem()

        # ... plot settings

        self.text = pg.TextItem('Loading...')

        self.view.addItem(self.text)      

        self.my_signal.connect(lambda: self.text.setText(self.textGen()))

        
        # ... other stuffs

        

        QtGui.QApplication.instance().exec_()

            

    # ... methods and stuffs

    

    def callback(self, ...):

        # ...

        self.my_signal.emit()

        

    def textGen(self):
        my_string = "..."

        return my_string

Thank you again.

Toni.