The difference between the usage of Qstackedwidget and qstackedlayout

import sys
from PyQt5 import QtWidgets, QtCore


class MyWidget (QtWidgets.QWidget):
    def __init __ (self, i = 0):
        super () .__ init __ ()
        self.setWindowTitle (‘Window‘ + str (i))
        label = QtWidgets.QLabel (‘label‘ + str (i))
        layout = QtWidgets.QHBoxLayout (self)
        layout.addWidget (label)
        

class TestWidget (QtWidgets.QWidget):
    def __init __ (self, parent = None):
        super () .__ init __ (parent)
        
        layout = QtWidgets.QHBoxLayout (self)
        
        #stack = QtWidgets.QStackedWidget () # ① Widget
        stack = QtWidgets.QStackedLayout () # ② Layout

        list = QtWidgets.QListWidget (self)
        list.setDragEnabled (True)
        list.setDragDropMode (QtWidgets.QAbstractItemView.InternalMove)

        layout.addWidget (list)
        # layout.addWidget (stack) # ① corresponding to addWidget
        layout.addLayout (stack) # ② corresponds to addLayout

        for i in range (10):
            stack.addWidget (MyWidget (i))
            list.addItem ("window% 04i"% i)

        list.currentRowChanged.connect (stack.setCurrentIndex)


if __name__ == "__main__":
    app = QtWidgets.QApplication (sys.argv)
    widget = TestWidget ()
    #widget = MyWidget ()
    widget.show ()
    sys.exit (app.exec_ ()) 

The difference between the usage of Qstackedwidget and qstackedlayout