How to destroy a singleton and then create a new one.

JonB

@a_fol
Well, it seems to be telling you to go del app before you can go app = qtw.QApplication(sys.argv) a second time. The answer at https://stackoverflow.com/a/6778997/489865 seems to be you (it says PyQt4, let's hope it applies to PyQt5 too).

Having said that, I do not know whether you ought also exit the exiting app "cleanly" from a Qt point of view. I don't know why one would want to destroy an existing QApplication and then recreate, but that's up to you.

a_fol

Thanks for your reply! I tried adding del app afte the first app.exec_() but I still get the same error about destroying the QApplication singleton before creating a new instance.

What I want is similar to this:
https://stackoverflow.com/questions/59581668/how-to-destroy-a-qapplication-and-then-run-a-new-one-without-exiting-the-python

But instead I can't have the two applications connected to each other as that prevents the functions between them to run in a sequence. So I can't think of another way other than destroying the first one and creating a new one after the functions in between are done.

J.Hilk

@a_fol
let me ask this, why do you want to destroy the QApplication to then immediately create a new one!?

What's stopping you from doing everything inside one QApplication instance?

a_fol

@a_fol said in How to destroy a singleton and then create a new one.:

RuntimeError: Please destroy the QApplication singleton before creating a new QApplication instance.

This error:

RuntimeError: Please destroy the QApplication singleton before creating a new QApplication instance.

If there is a way having both windows displayed in the sequence mentioned above, that is:

w1
Recognizer, SpeechConvert etc functions
w2
then I that would be great. It's just that looking at the error I thought destroying w1 and then creating w2 would solve everything.

jsulm

@a_fol There is really no need to recreate QApplication to show two windows in a sequence!
First show the first window. When it is closed create and show the second one.

canol

Hello, interestingly, I created almost the same question about the same time in another thread: https://forum.qt.io/topic/110416/pyside2-destroying-the-qapplication-instance-and-creating-another-one-gives-an-error

If I continue here; my problem is the same thing (I cannot create a new QApplication after destroying one) and my application is not a singleton, so users are able to quit the application and create a new one in their programs. I can make it a singleton, and maybe I should, since a process cannot have more than one QApplication running at the same time as far as I know.

But I still wonder why this code does not work, and if this is a bug? If yes, I'll look out for a way to report it. By the way the code works without a problem on PyQt5, it gives the singleton error only with PySide2.

For reference the code I try:

import sys
from PySide2.QtWidgets import *

app = QApplication(sys.argv)
label = QLabel("Hello World")
label.show()
app.exec_()

del app

app = QApplication(sys.argv)
label = QLabel("Hello World")
label.show()
app.exec_()

sys.exit()

Pablo J. Rogina

@canol said in How to destroy a singleton and then create a new one.:

Hello, interestingly, I created almost the same question about the same time in another thread: https://forum.qt.io/topic/110416/pyside2-destroying-the-qapplication-instance-and-creating-another-one-gives-an-error

Please don't double post!!

JonB

@canol
I was the person who replied to you in your other thread to come here to this one!

As per my observation there, your findings imply that there is a difference between PyQt5 & PySide2 here. Whether it;s a "bug" or "different handling" I can't say. It looks like the behaviour of del app varies. Not sure what to tell you to do if you insist on multiple QApplications, because the only answers are for PyQt5. You might try "shutting down" your first QApplication better, but I'm not certain how.....

canol

@Pablo-J-Rogina said in How to destroy a singleton and then create a new one.:

@canol said in How to destroy a singleton and then create a new one.:

Hello, interestingly, I created almost the same question about the same time in another thread: https://forum.qt.io/topic/110416/pyside2-destroying-the-qapplication-instance-and-creating-another-one-gives-an-error

Please don't double post!!

I did not double post, we created similar question almost at the same time, I did not see this poster's post before I created mine. Do you have anything useful to add to the discussion?

@JonB said in How to destroy a singleton and then create a new one.:

@canol
I was the person who replied to you in your other thread to come here to this one!

As per my observation there, your findings imply that there is a difference between PyQt5 & PySide2 here. Whether it;s a "bug" or "different handling" I can't say. It looks like the behaviour of del app varies. Not sure what to tell you to do if you insist on multiple QApplications, because the only answers are for PyQt5. You might try "shutting down" your first QApplication better, but I'm not certain how.....

Thank you, I'll investigate further.

sergio bartolini

@canol , I've met same issue in Spyder (debug is possible one time after start only).
So i've found a workaround:

    app = QApplication.instance()
    if app == None:
        app = QApplication([])

instead of usual

    app = QApplication([])

Maybe it'll help for you. It did the trick for me.

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How to destroy a singleton and then create a new one.