Debugging inconsistent QObject::setParent: Cannot set parent, new parent is in a different thread

gfinleyg · 18 Jun 2024, 05:09

I have a large project that I've been working on and I recently wanted to change a large QLabel into a series of rows of QLabels. I managed to get the class for it visually working the way I want it to, but inconsistently it crashes with only a few of the QLabels shown and a "QObject::setParent: Cannot set parent, new parent is in a different thread" error. Is there a way to see what components this error is related to exactly?

I think it must have something to do with the self.qlabels[i].setText(txt) or self.qlabels[i].show() lines in the class, but when I run it standalone I can't get it to return any errors or crashes. In the larger part of the project I am calling setFullText from a different thread, but I haven't run into this problem before when setting things from other classes.

I'm open to other ideas for moving things around or creating and deleting QLabels instead of hide/show, but this is the class code that came closest to working:

from PyQt6.QtWidgets import QLabel, QWidget, QVBoxLayout
#QWidget that reduces input text lines to fit a number of Qlabels in the widget's height
#adds "..." at the top if lines are reduced and top is true
#adds "...more" at the bottom if lines are reduced and top is false
class LongVLabels(QWidget):
    def __init__(self, top=False):
        super(LongVLabels,self).__init__()
        self.top = top #boolean storing if the top of the text is shown (True) or the bottom (False)
        self.fullText = ""
        #two layouts in the widget, one to measure available size and one to store added widgets
        self.layoutMain = QVBoxLayout()
        self.layoutH = QVBoxLayout()
        #start with 100 widgets (QLabels), hidden
        self.widgetCount = 100
        self.qlabels = [QLabel() for x in range(self.widgetCount)]
        for x in self.qlabels:
            x.setStyleSheet("qproperty-indent:8;background-color:#ffc5d1;border-radius: 6px;")
            x.setFixedSize(500,18)
            self.layoutH.addWidget(x)
            x.hide()
        self.layoutMain.addLayout(self.layoutH)
        self.setLayout(self.layoutMain)

    def make_reduced_lines_text(self):
        #get the current line space available
        availableHeight = self.geometry().height()
        #hide any existing qlabels
        for x in self.qlabels:
            x.hide()
        reducedText = ""
        maxLines = int(availableHeight/24)
        splitText = self.fullText.split("\n") #number of lines for the box
        lenSplitText = len(splitText)
        if splitText == [""]:
            reducedText = "Almost done!"
        elif splitText == ["DONE"]:
            reducedText = ""
        elif lenSplitText < maxLines:
            #all the text fits, no need to adjust line count
            reducedText = self.fullText
        else:
            #text lines don't fit, resize
            if self.top:
                reducedText = "\n".join(splitText[:maxLines-1]) + "\nmore...."
            else:
                start = lenSplitText - maxLines + 2 #extra line space for the ellipsis
                reducedText = "...\n" + "\n".join(splitText[start:])

        for i, txt in enumerate(reducedText.split("\n")):
            if i < self.widgetCount:
                self.qlabels[i].setText(txt)
                self.qlabels[i].show()

    def setFullText(self, textInput):
        self.fullText = textInput
        self.make_reduced_lines_text()

    def resizeEvent(self, event):
        self.make_reduced_lines_text()

I'm running Windows10 with Python v3.10.11 and PyQt6 v6.7.0.

jsulm · wrote on 18 Jun 2024, 05:09

@gfinleyg said in Debugging inconsistent QObject::setParent: Cannot set parent, new parent is in a different thread:

In the larger part of the project I am calling setFullText from a different thread

Don't do that! It is not supported. Only UI/main thread may access UI stuff.
If you want to trigger a change in the UI from another thread then emit a signal in that other thread and connect a slot in the main thread where you do the UI change.

gfinleyg · 18 Jun 2024, 08:43

Thanks for the quick response jsulm. I'll check out some other examples and see if I can figure out an easy way to interact with it using signals/slots.

Any ideas on the other question I had or is that info not easily exposed?
"QObject::setParent: Cannot set parent, new parent is in a different thread" error. Is there a way to see what components this error is related to exactly?

jeremy_k · wrote on 18 Jun 2024, 06:40

@gfinleyg said in Debugging inconsistent QObject::setParent: Cannot set parent, new parent is in a different thread:

I'm open to other ideas for moving things around or creating and deleting QLabels instead of hide/show, but this is the class code that came closest to working:

What's the motivation for this change? It sounds like a significant pessimization.

gfinleyg · 18 Jun 2024, 07:51

@jeremy_k I tried some other solutions that created the QLabel objects on the fly based on the number of lines rather than making 100 of them and hide/showing them selectively, that path seems to make more sense to me, but it also encountered the same crash earlier. It's only 57 lines of code anyway :)

jeremy_k · wrote on 18 Jun 2024, 07:51

@gfinleyg said in Debugging inconsistent QObject::setParent: Cannot set parent, new parent is in a different thread:

@jeremy_k I tried some other solutions that created the QLabel objects on the fly based on the number of lines rather than making 100 of them and hide/showing them selectively, that path seems to make more sense to me, but it also encountered the same crash earlier. It's only 57 lines of code anyway :)

It's not the creation and management of labels that I'm wondering about, but the decision to use more than one label (or QTextEdit/QPlainTextEdit/QTextBrowser) at all.

Lines of code isn't a good measure of runtime efficiency. while true: pass is only 1 line, but I'm still waiting for it to complete...

gfinleyg · 18 Jun 2024, 08:07

@jeremy_k I'm planning to set each QLabel's background color based on what was in the text. These won't be updated frequently while the code is running so I don't think the overhead will be that bad. I can always switch to the one QLabel with shortened text that I was using before if it turns out to be too cumbersome.

jeremy_k · wrote on 18 Jun 2024, 08:07

@gfinleyg said in Debugging inconsistent QObject::setParent: Cannot set parent, new parent is in a different thread:

@jeremy_k I'm planning to set each QLabel's background color based on what was in the text.

This sounds like a use for rich text (aka html). The syntax highlighter example may be of help.

gfinleyg · wrote on 18 Jun 2024, 08:11

@jeremy_k I was looking at that, I'm also partial to the oval shape around the text that you can get with a QLabel and some styling. That may be another fallback though, thanks for the suggestion.

jsulm · wrote on 18 Jun 2024, 08:43

@gfinleyg said in Debugging inconsistent QObject::setParent: Cannot set parent, new parent is in a different thread:

"QObject::setParent: Cannot set parent, new parent is in a different thread" error

Don't modify UI from other threads and you will not have this error...

gfinleyg · wrote on 18 Jun 2024, 09:00

Yep, I got it working by adding some signal handling. I suppose if I run into another vague error I can ask about this again. Thanks!