Using `qtpy.uic.loadUi` with PySide6 can lead to segmentation fault

[sipkema]

In an application that uses QtPy with PySide6 as the back-end / binding library, I occasionally got a segmentation fault. I was able to reproduce the error in a minimal example. I've already reported the issue on the Qt for Python bug tracker—it includes an attachment with the example code—but they closed it as being out of scope, so I figured I would at least let the QtPy maintainers know that this segmentation fault can happen.

Perhaps my usage of the qtpy.uic.loadUi function is incorrect and there is a way to use it with the baseinstance parameter without risking a segmentation fault?

[dalthviz]

Hi @sipkema thank you for the feedback and sorry for the late response! The loadUi implementation that we have in QtPy is quite old to be honest (is something that is supported as a built-in only for PyQt bindings and added to QtPy when Qt4 bindings were still supported) so is possible that using it with PySide6 bindings can cause troubles. From the comments you got in the Qt for Python issue I think a way to prevent the segfault (as mentioned in the comment there) is to not set a parent for the QUiLoader instance. In the case of the loadUi function available on QtPy, you will need to pass as baseinstance None or in the example for the crash you submitted to the Qt for Python issue tracker instead of passing as target the dialog instance just don't setup a dialog class and directly call QUiLoader().load and set the returned value to a variable (which should be a QDialog object). So using as example your main_window.py I think you will need to do something like:

import gc
import threading
import time
from importlib import resources

from PySide6 import QtWidgets
from PySide6.QtGui import QAction
from PySide6.QtUiTools import QUiLoader

class MainWindow(QtWidgets.QMainWindow):

    def __init__(self, parent=None):
        super().__init__(parent)

        self.showDialogAction = QAction("Show Dialog ...", self)

        self.menuBar().addAction(self.showDialogAction)

        self.showDialogAction.triggered.connect(
            self._onShowDialogActionTriggered)

    def _onShowDialogActionTriggered(self):
        with resources.path(__package__, "test_dialog.ui") as uiFile:
            dialog = QUiLoader().load(str(uiFile))
            dialog.exec()

        def task():
            time.sleep(1.0)
            gc.collect()

        threading.Thread(target=task).start()

Or using QtPy, something like:

import gc
import threading
import time
from importlib import resources

from qtpy import QtWidgets
from qtpy.QtGui import QAction
from qtpy.uic import loadUi

class MainWindow(QtWidgets.QMainWindow):

    def __init__(self, parent=None):
        super().__init__(parent)

        self.showDialogAction = QAction("Show Dialog ...", self)

        self.menuBar().addAction(self.showDialogAction)

        self.showDialogAction.triggered.connect(
            self._onShowDialogActionTriggered)

    def _onShowDialogActionTriggered(self):
        with resources.path(__package__, "test_dialog.ui") as uiFile:
            dialog = loadUi(str(uiFile))
            dialog.exec()

        def task():
            time.sleep(1.0)
            gc.collect()

        threading.Thread(target=task).start()

Let us know if the info above helps!

[sipkema]

Hi @dalthviz,

Thanks for your reply. I was using loadUi to define part of the user interface, but also using a Python class to define the behavior. I have now removed all use of the loadUi since it does not work for this use case, and it wasn't that convenient to begin with.

What would probably help someone like me—with the same not-so-great idea of defining part of the GUI in a .ui file—is to stop them before they put in much effort. I would rather always get an exception, instead of sometimes failing with a segmentation fault.

If passing anything other than None to baseinstance will not work for PySide6, you could consider throwing when loadUi is used incorrectly. Or maybe always trigger a deprecation warning, since using loadUi with baseinstance not None is not portable.