creating a desktop app window in python with pyqt

app.py

class App(QWidget):
    def __init__(self):
        super().__init__()
        ## Set main window attributes
        self.title = 'Photo Album Viewer'
        self.left = 0
        self.top = 0
        self.width = 800
        self.height = 600
        self.resizeEvent = lambda e : self.on_main_window_resize(e)

        ## Make 2 widgets, one to select an image and one to display an image
        self.display_image = DisplayImage(self)
        self.image_file_selector = ImageFileSelector( \
            album_path=DEFAULT_IMAGE_ALBUM_DIRECTORY, \
            display_image=self.display_image)
        scroll = QScrollArea()
        scroll.setWidgetResizable(True)
        scroll.setFixedWidth(140)
        nav = scroll
        nav.setWidget(self.image_file_selector)

        ## Add the 2 widgets to the main window layout
        layout = QGridLayout(self)
        layout.addWidget(nav, 0, 0, Qt.AlignLeft)
        layout.addWidget(self.display_image.label, 0, 1, Qt.AlignRight)

        self.init_ui()

    def init_ui(self):
        self.setWindowTitle(self.title)
        self.setGeometry(self.left, self.top, self.width, self.height)
        self.show()

    ## Set the display image size based on the new window size
    def on_main_window_resize(self, event):
        self.display_image.on_main_window_resize(event)


if __name__ == '__main__':
    app = QApplication(sys.argv)
    ex = App()
    sys.exit(app.exec_())