QStackedWidget 与 QStackedLayout 的用法区别

import sys
from PyQt5 import QtWidgets, QtCore

class MyWidget(QtWidgets.QWidget):
def __init__(self, i=0):
super().__init__()
self.setWindowTitle('窗口 ' + str(i))
label = QtWidgets.QLabel('标签 ' + 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 对应 addWidget
layout.addLayout(stack) # ② 对应 addLayout

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

# ... ...

self.contentsWidget.currentItemChanged.connect(self.changePage)
#self.contentsWidget.currentRowChanged.connect(self.pagesWidget.setCurrentIndex)

def changePage(self, current, previous):
if not current:
current = previous
self.pagesWidget.setCurrentIndex(self.contentsWidget.row(current))