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

Question

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?

Answer 1

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.

Answer 2

Brain's answer explains the problem perfectly. This Link explain things in more detail.

My solution to this problem was to set the widgets as attributes of the object (e.g. simply using self.label = ... instead of label = ... in your class methods). You might want to do the same for any layouts attached to the widget.

This way you create a copy of the widget so that when C++ memory cleanup occurs, you still have a reference to the widget.

Hope this helps.

Answer 3

In another case, the solution was to add all child objects to a detached layout first, and adding the layout to the parent layout as the last step. That is:

    l = QGridLayout()
    l.addWidget(QLabel("child1"), 0, 0)
    l.addWidget(QLabel("child2"), 0, 1)
    ...
    parentLayout.addLayout(l)