PyQt5 - Passed reference is None

Question

so I'm pretty new to Python. And for the life of me can't figure out why the following code isn't working (I'm using PyQt5). I'm basically trying to have 2 widgets inside a stackedwidget so I can switch between them. And the button to switch from window 0 to window 1 would be in window 0 obviously. So I would need to be able to somehow reference the stackedwidget. But when I try to pass the stackedwidget as a reference, it complains that the variable is None, even when that shouldn't be the case.

from PyQt5.QtWidgets import QApplication, QWidget, QDesktopWidget, QGridLayout, QPushButton, QHBoxLayout, QVBoxLayout, QStackedWidget
from PyQt5.QtCore import QCoreApplication

class DeviceSelectionWindow(QWidget):
    def __init__(self, mainWindow):
        super().__init__()
        self.initUI()

    def initUI(self):
         testB = QPushButton("test",self)
#------------------------------------------------------------
class ModeSelectionWindow(QWidget):

    def __init__(self, mainWindow):
        super().__init__()
        self.mainWindow = mainWindow
        self.initUI()

    def initUI(self):
        recordButton = QPushButton("Record tutorial")
        watchButton = QPushButton("Watch tutorial")

        recordButton.setFixedSize(200,100)
        recordButton.setStyleSheet("font-size:30px")
        watchButton.setFixedSize(200,100)
        watchButton.setStyleSheet("font-size:30px")
        recordButton.clicked.connect(self.mainWindow.setCurrentIndex(1))
        #Add horizontal strech layout box (centered)
        hbox = QHBoxLayout()
        hbox.addWidget(recordButton)
        hbox.addWidget(watchButton)


        #Add vertical strech layout box (centered) 
        vbox = QVBoxLayout()
        vbox.addLayout(hbox)

        self.setLayout(vbox)

#------------------------------------------------------------
class MainWindow(QStackedWidget):

    def __init__(self, **kwargs):
        super().__init__(**kwargs)
        self.initUI()


    def initUI(self):
        self.resize(1200,600)
        self.centerWindow()
        self.setWindowTitle("MultiPov Tutorial")

        modeSelectionWindow = ModeSelectionWindow(self)
        deviceSelectionWindow = DeviceSelectionWindow(self)

        self.addWidget(modeSelectionWindow)
        self.addWidget(deviceSelectionWindow)

        self.show()

    def centerWindow(self):
        qr = self.frameGeometry()
        cp = QDesktopWidget().availableGeometry().center()
        qr.moveCenter(cp)
        self.move(qr.topLeft())



#------------------------------------------------------------

if __name__=='__main__':
    app = QApplication(sys.argv)
    mainWindow = MainWindow()
    sys.exit(app.exec_())

Answer 1

The connect method takes a function name, you're doing a function call inside it which return None.

recordButton.clicked.connect(self.mainWindow.setCurrentIndex(1))

One way to get around this is to write a method that does the work (change the index)

def setWindow1(self):
    self.mainWindow.setCurrentIndex(1)

Then connect that method to the clicked signal

recordButton.clicked.connect(self.setWindow1)

Or, use a lambda function and get it done in one line

recordButton.clicked.connect(lambda: self.mainWindow.setCurrentIndex(1))