[FIXED] PyQt6 deleting custom widget with nested class causes the program to crash

Issue

I am using Python 3.9.5.

I have encountered some serious problem in my project and here is a minimum reproducible example code, along with some descriptions.

from PyQt6.QtCore import *
from PyQt6.QtGui import *
from PyQt6.QtWidgets import *

class Editor(QTextEdit):
    doubleClicked = pyqtSignal(QTextEdit)
    def __init__(self):
        super().__init__()
        self.setReadOnly(True)
    
    def mouseDoubleClickEvent(self, e: QMouseEvent) -> None:
        self.doubleClicked.emit(self)

class textcell(QGroupBox):
    def __init__(self, text):
        super().__init__()
        self.setAlignment(Qt.AlignmentFlag.AlignLeft | Qt.AlignmentFlag.AlignTop)
        self.label = QLabel(text)
        self.apply = makebutton('Apply')
        self.apply.hide()
        self.editor = Editor()
        self.editor.doubleClicked.connect(lambda: self.editor.setReadOnly(False))
        self.editor.doubleClicked.connect(self.apply.show)
        self.hbox = QHBoxLayout()
        self.hbox.addSpacerItem(spacer)
        self.hbox.addWidget(self.apply)
        self.vbox = QVBoxLayout()
        self.vbox.addWidget(self.label)
        self.vbox.addWidget(self.editor)
        self.vbox.addLayout(self.hbox)
        self.setLayout(self.vbox)
        self.apply.clicked.connect(self.on_ApplyClick)
    
    def on_ApplyClick(self):
        self.editor.setReadOnly(True)
        self.apply.hide()


def makebutton(text):
    button = QPushButton()
    button.setFixedSize(60, 20)
    button.setText(text)
    return button


class songpage(QGroupBox):
    def __init__(self, texts):
        super().__init__()
        self.init(texts)
        self.setCheckable(True)
        self.setChecked(False)
    
    def init(self, texts):
        self.vbox = QVBoxLayout()
        artist = textcell('Artist')
        artist.editor.setText(texts[0])
        album = textcell('Album')
        album.editor.setText(texts[1])
        title = textcell('Title')
        title.editor.setText(texts[2])
        self.vbox.addWidget(artist)
        self.vbox.addWidget(album)
        self.vbox.addWidget(title)
        self.setLayout(self.vbox)

spacer = QSpacerItem(0, 20, QSizePolicy.Policy.Expanding, QSizePolicy.Policy.Minimum)

class Ui_MainWindow(QMainWindow):
    def __init__(self):
        super().__init__()
        self.resize(405, 720)
        self.setWindowTitle('example')
        frame = self.frameGeometry()
        center = self.screen().availableGeometry().center()
        frame.moveCenter(center)
        self.move(frame.topLeft())
        self.centralwidget = QWidget(self)
        vbox = QVBoxLayout(self.centralwidget)
        hbox = QHBoxLayout()
        add = makebutton('Add')
        delete = makebutton('Delete')
        hbox.addWidget(add)
        hbox.addSpacerItem(spacer)
        hbox.addWidget(delete)
        vbox.addLayout(hbox)
        self.scrollArea = QScrollArea(self.centralwidget)
        self.scrollArea.setWidgetResizable(True)
        self.scrollAreaWidgetContents = QWidget()
        self.scrollArea.setVerticalScrollBarPolicy(Qt.ScrollBarPolicy.ScrollBarAlwaysOn)
        self.verticalLayout = QVBoxLayout(self.scrollAreaWidgetContents)
        self.verticalLayout.setAlignment(Qt.AlignmentFlag.AlignTop)
        self.scrollArea.setWidget(self.scrollAreaWidgetContents)
        self.scrollArea.setAlignment(Qt.AlignmentFlag.AlignLeft | Qt.AlignmentFlag.AlignTop)
        vbox.addWidget(self.scrollArea)
        self.setCentralWidget(self.centralwidget)
        add.clicked.connect(self.addObj)
        delete.clicked.connect(self.deleteObj)
    def addObj(self):
        Obj = songpage(('AAA', 'BBB', 'CCC'))
        self.verticalLayout.addWidget(Obj)
    def deleteObj(self):
        item = self.verticalLayout.itemAt(0)
        widget = item.widget()
        self.verticalLayout.removeItem(item)
        self.verticalLayout.removeWidget(widget)


app = QApplication([])
window = Ui_MainWindow()
window.show()
app.exec()

The problem is very simple, if I click add button, the widget will be added and everything works fine, if I double click on a QTextEdit, its apply button will show and it will change from read only to editable.

After I click an apply button, the button will hide and the corresponding QTextEdit will be read only again.

I have finally managed to add a doubleClicked signal to QTextEdit.

And, the problem, is if I click delete button, instead of deleting the thing as expected, it crashes the whole application.

I am sorry the minimum reproducible example is so long, but I have only managed to reproduce the issue with all the code and I really don't know what went wrong.

So how to fix it?

Solution

Well, I have figured it out, it was caused by the spacer item that I add to every one of horizontal layouts where fixed-sized buttons are used.

All such layouts are using the same spacer item, exactly the same spacer item, not just identical.

From what I have observed, the spacer items are all references to the same object, they are all echos of the same object which lives at a fixed memory address.

I honestly don't understand how such thing works, that the same object can not only be added to multiple layouts and present in all the layouts concurrently, but also be added to the same layout multiple times, yet it always remains the original object, not duplicates of itself.

I thought when I added the same spacer item to multiple layouts, I didn't add the original spacer item, instead duplicates of the original item which are identical but are at different memory addresses, and clearly that isn't how Python works.

So when I remove a widget, everything inside it, everything inside its layout is deleted, and the spacer item is deleted.

Because all the spacer items in all the layouts are references to the original spacer item, when I delete one of the layouts, the original spacer item is deleted as well, and the spacer item is deleted from all other layouts, yet its shadows remain, and the item isn't properly removed from all the other layouts, the layouts contain references to an object that no longer exists, thus the application crashed.

By removing the definition of the spacer item and replacing adding the spacer item with .addStretch(), the bug is fixed.

Answered By - Thyebri