Proper way to handle the close button in a main window PyQt, (Red "X")

You shouldn't ever modify the class that was generated from your ui file. Instead you should subclass and modify the subclass.

From the looks of your code, you are actually creating two QMainWindows and the closeEvent is being caught for the wrong one (presumably that one is hidden?). That is self.ui is a QMainWindow that is not being shown, and is not added to the UI of GUIForm. Instead you are using the Ui_MainWindow.setupUi() method explicitly yourself, to add the widgets to your own QMainWindow, 'GUIForm`.

Instead, what you should to do is leave your Ui_MainWindow class as it was when it was generated from the ui file, and then modify your main python file to be:

class GUIForm(Ui_MainWindow):
    def __init__(self, parent=None):
        Ui_MainWindow.__init__(self, parent)
        self.threadData()

    def closeEvent(self, event):
        print "User has clicked the red x on the main window"
        event.accept()


if __name__ == "__main__":

    app = QtGui.QApplication(sys.argv)
    myapp = GUIForm()
    myapp.show()
    ret = app.exec_()
    sys.exit(ret)

This way you are extending the behaviour of the auto-generated UI file. This makes it easy to regenerate the python file from the .ui file without having to re-add code (this is precisely why you should never modify the auto-generated Python file)

Sometimes, issues occur when handling event signals from the main window.

You can use the code :

app.aboutToQuit.connect(self.closeEvent)

and you can write your own code in the closeEvent function.

Note :

app is the name of the QtGui.QApplication instance

Here's a demo of the full code :

from PyQt4 import QtCore, QtGui

class Ui_MainWindow(object):
    def setupUi(self, MainWindow):
        MainWindow.setObjectName("MainWindow")
        MainWindow.resize(800, 600)
        self.centralwidget = QtGui.QWidget(MainWindow)
        self.centralwidget.setObjectName("centralwidget")
        MainWindow.setCentralWidget(self.centralwidget)
        self.retranslateUi(MainWindow)
        QtCore.QMetaObject.connectSlotsByName(MainWindow)

        #{================================

        app.aboutToQuit.connect(self.closeEvent)

        #}================================

    def retranslateUi(self, MainWindow):
        MainWindow.setWindowTitle('Demo')

    #{================================

    def closeEvent(self):
        #Your desired functionality here
        print('Close button pressed')
        import sys
        sys.exit(0)

    #}================================


if __name__ == "__main__":
    import sys
    app = QtGui.QApplication(sys.argv)
    MainWindow = QtGui.QMainWindow()
    ui = Ui_MainWindow()
    ui.setupUi(MainWindow)
    MainWindow.show()
    sys.exit(app.exec_())

Hope it helps.

I ran into this same problem, where I had a dialog window prompt, asking the user for input before proceeding in an operation. The user could add input before the operation was processed, skip adding input and process anyway or cancel out of the operation using a Cancel button or by clicking the X of the dialog window.

What I did instead was create a variable holding a boolean value that would be checked before closing the window, so that in the event the window was forced closed, it's clear that a process was aborted.

class Ui_MainWindow(QtGui.QMainWindow):
    def __init__(self):
        QtGui.QMainWindow.__init__(self)
        self.setupUi(self)
        #This variable will be your way of determining how the window was closed
        self.force_close = True
        self.buttonOk.clicked.connect(self.someOtherFunction)

    def setupUi(self, MainWindow):
        #setup code goes here

    def retranslateUi(self, MainWindow):
        #re translation of the GUI code 

    def someOtherFunction(self):
        # Do some things here if needed
        print('All done')
        # Assuming some operation is performed and a value or result is generated            
        # Since this function completed, there is no need to force close the window
        # So it can be set to False
        self.force_close = False
        # the below close() is a built-in function and will automatically trigger the
        # closeEvent
        self.close()

    def closeEvent(self, event):            
        if self.force_close is True:
            # Here is where you could add you code to perform some operation
            # due to the user clicking the X