main_window.py

from PySide2.QtCore import *
from PySide2.QtCore import Slot, Signal
from PySide2.QtWidgets import *

def qt_splitter(
        *objects,
        orientation: Qt.Orientation = Qt.Vertical,
        sizes: list = None
) -> QSplitter:
    splitter = QSplitter(orientation)
    for obj in objects:
        if isinstance(obj, QWidget):
            splitter.addWidget(obj)
        if isinstance(obj, QLayout):
            wdg = QWidget()
            wdg.setLayout(obj)
            splitter.addWidget(wdg)

        if sizes is not None:
            splitter.setSizes(sizes)

    return splitter


class MainWindow(QMainWindow):

    def __init__(self, parent=None):
        super().__init__(parent)

        # Actions
        open_file_act: QAction = QAction(self.tr("Open file..."))
        open_file_act.triggered.connect(self.__open_file_slot)

        # Menu
        self.menubar: QMenuBar = QMenuBar(self)
        #
        file_menu: QMenu = QMenu(self.tr("File"), self.menubar)
        file_menu.addAction(open_file_act)
        #
        self.menubar.addMenu(file_menu)

        # Toolbar
        self.toolbar: QToolBar = QToolBar(self)
        self.toolbar.toggleViewAction().setText(self.tr("Show/hide tool bar"))
        self.toolbar.setMovable(False)
        #
        self.toolbar.addAction(open_file_act)

        self.navigation: QTreeWidget = QTreeWidget(self)

        self.container: QStackedWidget = QStackedWidget(self)

        self.statusbar: QStatusBar = QStatusBar(self)

        # Link
        self.setMenuBar(self.menubar)
        self.addToolBar(self.toolbar)
        self.setCentralWidget(
            qt_splitter(
                self.navigation,
                self.container,
                orientation=Qt.Horizontal,
                sizes=[200, 1000]
            )
        )
        self.setStatusBar(self.statusbar)

    @Slot()
    def __open_file_slot(self):
        pass