How to stop qt app from freezing the main program?

Question

For example:

#!/usr/bin/env python3

import sys
from PySide import QtCore, QtGui

class Dialog(QtGui.QDialog):
    def __init__(self):
        QtGui.QDialog.__init__(self)
        button = QtGui.QPushButton("test")
        layout = QtGui.QVBoxLayout()
        layout.addWidget(button)
        self.setLayout(layout)

app = QtGui.QApplication(sys.argv)
toast = Dialog()
toast.show()
app.exec_()
print("App freezes the main process!")

The last print() function will not be executed until you close the dialog.

I am working on a script that only uses qt for displaying some content that does not require user interaction, so I would prefer the gui code runs in background.

Answer 1

I'm not sure if the PySide imposes any restrictions, but here's how it's done in C++:

1. Instantiate QApplication in a secondary thread.
2. Create your dialog in that same thread.
3. Call either QDialog::exec() OR {QApplication::exec() plus QDialog::show()} in that same thread.
4. Make sure that your secondary thread has fully shut down before you quit your app.

---

Yes, the Qt documentation currently says that only the main thread is allowed. However, there is nothing in the Qt source code that forbids creating QApplication in a secondary thread and then using GUI classes in that thread (for Windows and Linux). The documentation should be changed.

Mac OS X is different though -- the Cocoa framework only allows GUI operations in the main thread.

Answer 2

This is not possible. Qt documentation states:

Although QObject is reentrant, the GUI classes, notably QWidget and all its subclasses, are not reentrant. They can only be used from the main thread. As noted earlier, QCoreApplication::exec() must also be called from that thread.

^{(emphasis mine)}

This answer suggests on the other hand that in reality this is not true :) However it seems that PySide sticks to the official version:

This can be verified by the following code sample:

import sys
import threading
from PySide import QtCore, QtGui

class Dialog(QtGui.QDialog):
    def __init__(self):
        QtGui.QDialog.__init__(self)
        button = QtGui.QPushButton("test")
        layout = QtGui.QVBoxLayout()
        layout.addWidget(button)
        self.setLayout(layout)

app = QtGui.QApplication(sys.argv)
toast = Dialog()
toast.show()

t = threading.Thread(target = lambda: app.exec_())
t.daemon = True
t.start()
print("App freezes the main process!")
input()

which produces the following output:

App freezes the main process!
QApplication::exec: Must be called from the main thread

(and a crash, on my machine). I have also verified the option with creating the app within the other thread - it works, but crashes on exit.

---

So the solution seems to let Qt have the main thread, and organize your processing in a separate thread. This shouldn't really be a problem: if you'll separate your concerns well it won't make a difference for your console-app part on which thread it's running.