Error "QWidget: Must construct a QApplication..." with flowcharts

[michael.m...@gmail.com]

Hello,

I am interested in PyQtGraph's flowchart feature and wanted to try it on my system (Linux Mint 15).
I installed the Deb package from the website (python-pyqtgraph_0.9.7-1_all.deb).

I copy-pasted in the Python console the first flowchart example from the documentation ( http://www.pyqtgraph.org/documentation/flowchart/index.html ):

from pyqtgraph.flowchart import Flowchart
fc = Flowchart(terminals={
    'nameOfInputTerminal': {'io': 'in'},
    'nameOfOutputTerminal': {'io': 'out'}
})

And I get the following error after confirming the second line:
QWidget: Must construct a QApplication before a QPaintDevice

Any idea on how this can be fixed ?

Thank you for your help !
M. Mestre

[Guillaume Poulin]

 That's come from PyQt. Before creating any QWidget (PyQtGraph create many QWidget), you need to create to a QApplication and start the Qt event loop. This can be done by calling 

app=QtGui.QApplication([])
app.exec_()

This generally go in the "if __name__ == '__main__':" part of your python script. Be careful, creating two QApplication result in an Segfault. You can test if there is a QAppliction running with QtGui.QApplication.instance()

If you want to work in an interactive python console I suggest you to use IPython and to launch it with `ipython --pylab=qt`. IPython gonna take care to create the QApplication and gonna handle the Qt even loop.

I suggest you to look at a PyQt tutorial for more details.

[Guillaume Poulin]

Le dimanche 29 septembre 2013 18:47:30 UTC+8, Guillaume Poulin a écrit :

Le dimanche 29 septembre 2013 17:55:20 UTC+8, michael.m...@gmail.com a écrit :

Hello,

I am interested in PyQtGraph's flowchart feature and wanted to try it on my system (Linux Mint 15).
I installed the Deb package from the website (python-pyqtgraph_0.9.7-1_all.deb).

I copy-pasted in the Python console the first flowchart example from the documentation ( http://www.pyqtgraph.org/documentation/flowchart/index.html ):

from pyqtgraph.flowchart import Flowchart
fc = Flowchart(terminals={
    'nameOfInputTerminal': {'io': 'in'},
    'nameOfOutputTerminal': {'io': 'out'}
})

And I get the following error after confirming the second line:
QWidget: Must construct a QApplication before a QPaintDevice

Any idea on how this can be fixed ?

Thank you for your help !
M. Mestre

 That's come from PyQt. Before creating any QWidget (PyQtGraph create many QWidget), you need to create to a QApplication and start the Qt event loop.

In fact, you don't need a Qt event loop to create a QWidget but you are gonna need it for a lot of other things (like interacting with your graph).

[michael.m...@gmail.com]

Yup, it works :)

Thank you very much for your help !!
Are there any simple examples online for a minimal working program implementing flowcharts ?

Best regards,
Michael

[Guillaume Poulin]

Le dimanche 29 septembre 2013 18:55:23 UTC+8, michael.m...@gmail.com a écrit :

Yup, it works :)

Thank you very much for your help !!
Are there any simple examples online for a minimal working program implementing flowcharts ?

Best regards,
Michael

Did you have a look at 'examples/Flowchart.py' and 'examples/FlowchartCustomNode.py' included with PyQtGraph?

[michael.m...@gmail.com]

Perfect, thank you !!!

On Sunday, September 29, 2013 11:55:20 AM UTC+2, michael.m...@gmail.com wrote: