QMessageBox in other thread

Question

import time
import sys, threading
from PyQt4 import QtGui, QtCore
from PyQt4.QtGui import QApplication


class Global:
    def __init__(self):
        for c in MyClass, MainWindow:
            cl = c()
            setattr(self, c.__name__, cl)
            setattr(cl, 'global_', self)


class MyClass:
    def work(self):
        for r in range(100):
            if r == 2:
                self.global_.MainWindow.SignalBox.emit('MyClass NO PAUSE')  # need pause !!!
            else:
                print(r)
            time.sleep(1)


class MainWindow(QtGui.QWidget):
    Signal = QtCore.pyqtSignal(str)
    SignalBox = QtCore.pyqtSignal(str)

    def __init__(self):
        QtGui.QWidget.__init__(self)
        self.resize(300, 300)

        self.lab = QtGui.QLabel()
        lay = QtGui.QGridLayout()
        lay.addWidget(self.lab)
        self.setLayout(lay)

        self.msgBox = lambda txt: getattr(QtGui.QMessageBox(), 'information')(self, txt, txt)
        self.Signal.connect(self.lab.setText)
        self.SignalBox.connect(self.msgBox)

    def thread_no_wait(self):
        self.global_.MainWindow.SignalBox.emit('MyClass PAUSE OK')
        threading.Thread(target=self.global_.MyClass.work).start()

    def thread_main(self):
        def my_work():
            for r in range(100):
                self.Signal.emit(str(r))
                time.sleep(1)
        threading.Thread(target=my_work).start()


if __name__ == '__main__':
    app = QApplication(sys.argv)
    g = Global()
    g.MainWindow.show()
    g.MainWindow.thread_main()
    g.MainWindow.thread_no_wait()
    app.exec_()

how to run a QMessageBox from another process?

MyClass.work performed in a separate thread without join is necessary to MyClass suspended if it causes QMessageBox If we define QMessageBox is the MainWindow , the error will be called

QObject :: startTimer: timers can not be started from another thread QApplication: Object event filter can not be in a different thread. And in this call QMessageBox invoked in a separate thread , and the work is not suspended

Answer 1

It sounds like you want to pause execution of a thread until certain operations in the main thread complete. There are a few ways you can do this. A simple way is to use a shared global variable.

PAUSE = False

class MyClass:
    def work(self):
        global PAUSE
        for r in range(100):
            while PAUSE:
                time.sleep(1)
            if r == 2:
                self.global_.MainWindow.SignalBox.emit('MyClass NO PAUSE')
                PAUSE = True
            ...

class MainWindow(...)

    def msgBox(self, txt):
        QtGui.QMessageBox.information(self, txt, txt)
        global PAUSE
        PAUSE = False

If you use QThreads instead of python threads, you can actually call functions in another thread and block until that function completes using QMetaObject.invokeMethod

@QtCore.pyqtSlot(str)
def msgBox(self, txt):
    ...

...

if r == 2:
    # This call will block until msgBox() returns
    QMetaObject.invokeMethod(
        self.global_.MainWindow, 
        'msgBox', 
        QtCore.Qt.BlockingQueuedConnection, 
        QtCore.Q_ARG(str, 'Text to msgBox')
    )