Accessing QMainWindow class variables - Pyside/PyQt

Question

I'm writing a GUI application in Python, which uses multiple .py scripts. I have a variable in the QMainWindow, which I need to refer to/access in other classes. I don't have a problem importing the various .py modules into the Ui_MainWindow.py module, but I cannot seem to access the QMainWindow class variables.

This is a quick pseudo-code of what I'm trying:

class MainWindow(QMainWindow, Ui_MainWindow):
    def __init__(self, parent=None):
        super(MainWindow, self).__init__(parent)
        self.setupUi(self)

        self.lineEditScanBarcode.returnPressed.connect(self.LoginAttempt)

    def LoginAttempt(self):
        self.user_barcode = self.lineEditScanBarcode.text()

From the reading I've done on this referring to class variables, I've come to the conclusion that with the above setup, I should be able to refer to the 'user_barcode' variable in other classes as follows:

class Receipt(QWidget, Ui_Receipt):
    def __init__(self, parent=None):
        QWidget.__init__(self, parent)
        self.setupUi(self)

        print(MainWindow.user_barcode)

I've been using the 'print' command just to test whether it's working, but I receive the following error:

Attribute Error: type object 'MainWindow' has no attribute 'user_barcode'

Can anyone see the error I'm obviously making? I've searched SO for similar queries, but haven't found anything relevant.

Thanks!

EDIT:

Here's the app.exec_() setup, I'm not sure if I'm passing the parent correctly.

if __name__ == '__main__':
    app = QApplication(sys.argv)
    showMainWindow = MainWindow()
    showReceipt = Receipt(MainWindow)
    showMainWindow.show()
    app.exec_()

I've tried various combinations, but I'm either receiving the init error, or the raised TypeError.

Answer 1

The reason the example code doesn't work, is because MainWindow is a class , whereas user_barcode is an attribute of an instance of that class.

For a Receipt to access the user_barcode attribute, it must somehow have the MainWindow instance made available to it. And one way to do that, is to set a MainWindow as the Receipt's parent.

This will then allow the Receipt to use the parent method to access the MainWindow instance and its attributes. Of course, this means a Receipt must always have a MainWindow as it's parent, so its constructor should probably look more like this:

class Receipt(QWidget, Ui_Receipt):
    def __init__(self, parent):
        if not isinstance(parent, MainWindow):
            raise TypeError('parent must be a MainWindow')
        super(Receipt, self).__init__(parent)
        self.setupUi(self)
        ...
        print(self.parent().user_barcode)