PyQt: RuntimeError: wrapped C/C++ object has been deleted

If I run this code:

    #!/usr/local/bin/    python3

import sys 
from PyQt4.QtCore import *
from PyQt4.QtGui import *


class Window(QMainWindow):

    def __init__(self):
        super().__init__()
        self.button1 = QPushButton("1")
        self.button2 = QPushButton("2")
        self.setCentralWidget(self.button1)
        self.button1.clicked.connect(lambda: self.setCentralWidget(self.button2))
        self.button2.clicked.connect(lambda: self.setCentralWidget(self.button1))
        self.show()

if __name__ == '__main__':

    import sys 
    app = QApplication(sys.argv)
    window = Window()
    sys.exit(app.exec_())

...I get this output:

Traceback (most recent call last):
  File "test.py", line 16, in <lambda>
    self.button2.clicked.connect(lambda: self.setCentralWidget(self.button1))
RuntimeError: wrapped C/C++ object of type QPushButton has been deleted

I do not understand why the object is being deleted. Window should maintain a reference to it. I have thoroughly investigated these posts: Understanding the “underlying C/C++ object has been deleted” error Can a PyQt4 QObject be queried to determine if the underlying C++ instance has been destroyed?

Why is the button being deleted?

This answer to this question is as found here: Python PySide (Internal c++ Object Already Deleted)

Apparently, assigning one widget to QMainWindow using setCentralWidget and then assigning another widget with setCentralWidget will cause the underlying c++ QWidget to be deleted, even though I have an object that maintains reference to it.

Note: QMainWindow takes ownership of the widget pointer and deletes it at the appropriate time.