Cfrgtkky

I have a main class that displays a window with a push button. On clicking the push button calls a child class (that contains while loop) that displays "Yay" in a separate window. I am not sure why the child window is displayed after executing the while loop. Attached is the code for your reference. Any help is much appreciated.

class test1(QtWidgets.QMainWindow):



    def __init__(self):

        super (test1, self).__init__()

        test1.setGeometry(self, 20, 20, 400, 250)



        btn = QtWidgets.QPushButton("Add", self)

        btn.clicked.connect(self.Check)    

        btn.resize(btn.sizeHint())

        btn.move(10, 170)   

        self.show()



    def Check(self):

        self.test2=test2()







class test2(QtWidgets.QMainWindow):



    def __init__(self):

        super (test2, self).__init__()



        test2.setGeometry(self,20, 20, 450, 100)

        self.label = QtWidgets.QLabel("Yay",self)

        self.show()

        a=50

        b=60

        x=True

        time.sleep(5)

        while (x):

            if a!=b:

                a+=1

            else:

                x=False

def run():

    app = QtWidgets.QApplication(sys.argv)

    GUI = test1()

    GUI.show()

    sys.exit(app.exec_())



run()

The GUI lives in an event-loop, this a loop that allows to obtain data of diverse elements like the keyboard, the mouse, the logic, etc and to update the status of the GUI, so if the GUI is frozen it will not allow to do its I work correctly. And in this case the one that blocks the event-loop is the sleep, the while does not intervene since it is executed in less than 1 ms.

So the tasks that take up a lot of time must be executed in a different thread of the GUI avoiding that it is blocked, but in your case the sleep can be replaced by a QTimer.

In conclusion, the sleep is freezing the GUI. I see that the code has an educational purpose so I will show you several options:

threading.Thread():

import threading



# ...



class test2(QtWidgets.QMainWindow):

    def __init__(self):

        super (test2, self).__init__()

        test2.setGeometry(self,20, 20, 450, 100)

        self.label = QtWidgets.QLabel("Yay",self)

        self.show()



        threading.Thread(target=self.foo, daemon=True).start()



    def foo(self):

        time.sleep(5)

        a=50

        b=60

        x=True

        while x:

            if a != b:

                a += 1

                print(a)

            else:

                x = False

QTimer + QEventLoop:

class test2(QtWidgets.QMainWindow):

    def __init__(self):

        super (test2, self).__init__()

        test2.setGeometry(self,20, 20, 450, 100)

        self.label = QtWidgets.QLabel("Yay",self)

        self.show()

        loop = QtCore.QEventLoop()

        QtCore.QTimer.singleShot(5*1000, loop.quit)

        loop.exec_()

        a=50

        b=60

        x=True

        while x:

            if a != b:

                a += 1

                print(a)

            else:

                x = False

QTimer:

class test2(QtWidgets.QMainWindow):

    def __init__(self):

        super (test2, self).__init__()

        test2.setGeometry(self,20, 20, 450, 100)

        self.label = QtWidgets.QLabel("Yay",self)

        self.show()

        QtCore.QTimer.singleShot(5*1000, self.foo)



    def foo(self):

        a=50

        b=60

        x=True

        while x:

            if a != b:

                a += 1

                print(a)

            else:

                x = False

As a general conclusion, the tasks executed in the thread of the GUI should consume a time less than 30ms, if it is greater than that it is recommended to execute it in another thread. In the case that each task within the loop fulfills the above but there are many iterations, a QTimer can be used to perform the tasks of the loop periodically.