Subclass QUiLoader

3beezer

I am subclassing QUiLoader so that I can control the widgets created in createWidget. My code is something like this:

from importlib import import_module

class MyLoader(QUiLoader):
    def __init__(self):
        super().__init__()
        self.widget_mod = import_module("PySide2.QtWidgets")

    def createWidget(self, class_name, parent=None, name=''):
        orig_widget_inst = QUiLoader.createWidget(self, class_name, parent=None, name='')
        my_widget_inst = getattr(self.widget_mod, class_name)(parent)
        return my_widget_inst

my_widget_inst appears to be equivalent to orig_widget_inst, but mine causes a segmentation fault. What else do I need to do to make my_widget_inst usable?

SGaist

Hi,

@3beezer said in Subclass QUiLoader:

def createWidget(self, class_name, parent=None, name=''):
orig_widget_inst = QUiLoader.createWidget(self, class_name, parent=None, name='')
widget_widget_inst = getattr(self.widget_mod, class_name)(parent)
return my_widget_inst

You are talking about my_widget_inst in your post but you don't have anything named like that created in your code. Is it just a typo ?

3beezer

@sgaist Typo. I fixed it. Sorry.

SGaist

Can you add a minimal sample code that uses this class and triggers the issue ?

3beezer

test.py

import sys
from importlib import import_module

from PySide2.QtCore import QFile
from PySide2.QtUiTools import QUiLoader
from PySide2.QtWidgets import QApplication

class MyLoader(QUiLoader):
    def __init__(self):
        super().__init__()
        self.widget_mod = import_module("PySide2.QtWidgets")

    def createWidget(self, class_name, parent=None, name=''):
        orig_widget_inst = QUiLoader.createWidget(self, class_name, parent=None, name='')
        print("orig_widget_inst =", orig_widget_inst)

        widget_class = getattr(self.widget_mod, class_name)
        my_widget_inst = widget_class(parent)
        print("my_widget_inst =", my_widget_inst)

        return orig_widget_inst
        #return my_widget_inst

if __name__ == "__main__":
    app = QApplication(sys.argv)
    ui_file = QFile(sys.argv[1])
    ui_file.open(QFile.ReadOnly)
    loader = MyLoader()
    loader.load(ui_file)
    ui_file.close()

and test.ui

<?xml version="1.0" encoding="UTF-8"?>
<ui version="4.0">
 <class>test_panel</class>
 <widget class="QWidget" name="test_panel"/>
 <resources/>
 <connections/>
</ui>

Run the program using

python test.py test.ui

Switch the comment on the two return statements to see the crash.

3beezer

I'm new here. Should I report this problem as a bug? If so, where?

SGaist

What version of PySide2 are you currently using ?
On what OS ?

PySide2 5.11.1 and 5.13.0 on macOS are working with your test example.

3beezer

@sgaist Interesting! I am using version 5.12.4 on Kubuntu 19.04 with Python 3.7.3. Oh, I see that I can upgrade PySide2 to 5.13.0 using pip. I will do so and report back.

3beezer

Nope. Still crashes. You did switch the # to the other return, right?

SGaist

No I didn't because I overlooked it and I must say I expect to run crashing examples with the "working" version in comment rather than the contrary.

Before digging further, can you just explain what you want to do exactly in createWidget ?

3beezer

I would like to wrap the widget and return an instance of the wrapped widget. Since I can't return an instance of the exact same widget without getting a crash, it doesn't seem to be possible to override createWidget, despite claims to the contrary.

SGaist

What wrapping do you have in mind ?

3beezer

I have several ideas. One I would have liked to implement earlier today is a subclass with __del__ so that I can confirm that I haven't created a cyclic reference. I do this in portions of the GUI I design manually, but I can't do it in portions I design using Qt Designer.

SGaist

I wonder if it's a python specific issue.

Can you test if the same issue happens in C++ ?

3beezer

I think it is, but I don't know C++ well enough to write a test program.

3beezer

The problem is officially a bug:

bug 1070