from PySide6.QtWidgets import QApplication, QWidget, QMainWindow, QDockWidget
from PySide6.QtCore import *
# Only needed for access to command line arguments
import sys

# You need one (and only one) QApplication instance per application.
# Pass in sys.argv to allow command line arguments for your app.
# If you know you won't use command line arguments QApplication([]) works too.

class MainWindow(QMainWindow):
    def __init__(self):
        QMainWindow.__init__(self)
        self.initUI()

    def initUI(self):
        # Central widget is not used.
        dummyWidget = QWidget()
        dummyWidget2 = QWidget()
        self.setCentralWidget(dummyWidget)
        self.dockWidget = QDockWidget()
        self.dockWidget.setAllowedAreas(Qt.RightDockWidgetArea | Qt.LeftDockWidgetArea)
        self.dockWidget.setWidget(dummyWidget2)
        self.addDockWidget(Qt.LeftDockWidgetArea, self.dockWidget)

app = QApplication(sys.argv)
ex = MainWindow()
app.processEvents()
ex.setVisible(True)

# Start the event loop.
app.exec_()

# Your application won't reach here until you exit and the event
# loop has stopped.