Question

I nearly finished my application, when the customer asked if I could implement some kind of login form on application startup.

So far I have designed the UI, and tinkered about the actual execution. Username and password are irrelevant for now.

class Login(QtGui.QDialog):
    def __init__(self,parent=None):
        QtGui.QWidget.__init__(self,parent)
        self.ui=Ui_dlgLogovanje()
        self.ui.setupUi(self)

        QtCore.QObject.connect(self.ui.buttonLogin, QtCore.SIGNAL("clicked()"), self.doLogin)

    def doLogin(self):
        name = str(self.ui.lineKorisnik.text())
        passwd = str(self.ui.lineSifra.text())
        if name == "john" and passwd =="doe":
            self.runIt()
        else:
            QtGui.QMessageBox.warning(self, 'Greška',
        "Bad user or password", QtGui.QMessageBox.Ok)           

    def runIt(self):
        myprogram = Window()        
        myprogram.showMaximized() #myprogram is

class Window(QtGui.QMainWindow):
    def __init__(self,parent=None):
        QtGui.QWidget.__init__(self,parent)
        self.ui=Ui_MainWindow()
        self.ui.setupUi(self)


if __name__=="__main__":
    program = QtGui.QApplication(sys.argv)
    myprogram = Window()
    if Login().exec_() == QtGui.QDialog.Accepted:       
        sys.exit(program.exec_())

Login form is shown. If correct username and password are entered, then main window is shown and working. But, the login form stays active, and if I close it, the main window will also close.

Answer 1

A QDialog has its own event loop, so it can be run separately from the main application.

You just need to check the dialog’s return code to decide whether the main application should be run or not.

Example code (PyQt5):

from PyQt5 import QtWidgets
# from mainwindow import Ui_MainWindow

class Login(QtWidgets.QDialog):
    def __init__(self, parent=None):
        super(Login, self).__init__(parent)
        self.textName = QtWidgets.QLineEdit(self)
        self.textPass = QtWidgets.QLineEdit(self)
        self.buttonLogin = QtWidgets.QPushButton('Login', self)
        self.buttonLogin.clicked.connect(self.handleLogin)
        layout = QtWidgets.QVBoxLayout(self)
        layout.addWidget(self.textName)
        layout.addWidget(self.textPass)
        layout.addWidget(self.buttonLogin)

    def handleLogin(self):
        if (self.textName.text() == 'foo' and
            self.textPass.text() == 'bar'):
            self.accept()
        else:
            QtWidgets.QMessageBox.warning(
                self, 'Error', 'Bad user or password')

class Window(QtWidgets.QMainWindow):
    def __init__(self, parent=None):
        super(Window, self).__init__(parent)
        # self.ui = Ui_MainWindow()
        # self.ui.setupUi(self)

if __name__ == '__main__':

    import sys
    app = QtWidgets.QApplication(sys.argv)
    login = Login()

    if login.exec_() == QtWidgets.QDialog.Accepted:
        window = Window()
        window.show()
        sys.exit(app.exec_())

Example code (PyQt4):

from PyQt4 import QtGui
# from mainwindow import Ui_MainWindow

class Login(QtGui.QDialog):
    def __init__(self, parent=None):
        super(Login, self).__init__(parent)
        self.textName = QtGui.QLineEdit(self)
        self.textPass = QtGui.QLineEdit(self)
        self.buttonLogin = QtGui.QPushButton('Login', self)
        self.buttonLogin.clicked.connect(self.handleLogin)
        layout = QtGui.QVBoxLayout(self)
        layout.addWidget(self.textName)
        layout.addWidget(self.textPass)
        layout.addWidget(self.buttonLogin)

    def handleLogin(self):
        if (self.textName.text() == 'foo' and
            self.textPass.text() == 'bar'):
            self.accept()
        else:
            QtGui.QMessageBox.warning(
                self, 'Error', 'Bad user or password')

class Window(QtGui.QMainWindow):
    def __init__(self, parent=None):
        super(Window, self).__init__(parent)
        # self.ui = Ui_MainWindow()
        # self.ui.setupUi(self)    

if __name__ == '__main__':

    import sys
    app = QtGui.QApplication(sys.argv)
    login = Login()

    if login.exec_() == QtGui.QDialog.Accepted:
        window = Window()
        window.show()
        sys.exit(app.exec_())