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Type: Bug
Resolution: Fixed
Priority: P3: Somewhat important
Fix Version/s: 6.5.1, 6.6.0
Affects Version/s: 5.15.10, 6.5.0
Component/s: PySide
Labels:
None

Platform/s:

Windows
Commits:
14299e5f5 (dev), 619a44493 (6.5)

Description

I have a widget that uses a QThreadPool to load QImage objects, then sends it back to the widget. It works in Python 2, but completely locks up in Python 3 until all images are loaded. I narrowed it down to to creating a QImage inside the QThreadPool. It works in Python 3 when using PyQt6, so I believe the issue is something specific to PySide.

Here's my tests on different versions:

[works] Python 2, PySide 1.2.4
[works] Python 2, PySide2 5.12.5
[fails] Python 3, PySide2 5.15.0
[fails] Python 3, PySide6 6.5.0
[works] Python 3, PyQt6

I have an MRE that demonstrates this (from my 2 year old SO question). If using Python 2 or PyQt6, the scrolling is smooth and threads run in the background while resizing or scrolling. If using PySide in Python 3, the window freezes until all threads are complete.

PATH_TO_LARGE_FILE = 'C:/large_image.png'  # Pick something >1MB to really show the slowness

import sys
from Qt import QtCore, QtGui, QtWidgets

class GridView(QtWidgets.QListView):
    def __init__(self):
        super(GridView, self).__init__()
        self.setViewMode(QtWidgets.QListView.IconMode)
        self.setModel(GridModel())

class GridModel(QtGui.QStandardItemModel):
    def __init__(self):
        super(GridModel, self).__init__()
        self.threadPool = QtCore.QThreadPool()

    def data(self, index, role=QtCore.Qt.UserRole):
        # Load image
        if role == QtCore.Qt.DecorationRole:
            item = self.itemFromIndex(index)
            worker = ImageLoader(item)
            self.threadPool.start(worker)
            return None

        # Set size of icons
        elif role == QtCore.Qt.SizeHintRole:
            return QtCore.QSize(64, 89)
        return super(GridModel, self).data(index, role)

class ImageLoader(QtCore.QRunnable):
    def __init__(self, item):
        self.item = item
        super(ImageLoader, self).__init__()

    def run(self):
        QtGui.QImage(self.item.path)

if __name__ == '__main__':
    app = QtWidgets.QApplication(sys.argv)
    widget = GridView()
    for i in range(1000):
        item = QtGui.QStandardItem('test {}'.format(i))
        item.path = PATH_TO_LARGE_FILE
        widget.model().appendRow(item)
    widget.show()
    app.setActiveWindow(widget)
    app.exec_()