PyQt5; problem with threading using QRunnable and QThreadPool

Question

It seems that I have a problem with running threads in my application as follows: in my MainForm class I have:

.
.
self.threadPool = QtCore.QThreadPool()
self.threadPool.setMaxThreadCount(4)
.
.
def openFileFcn(self):
worker = **Worker**(self, **self.ImageViewer.showImages**, self.files)
self.threadPool.start(worker)

where Worker is a wrapper QRunnable defined as:

class Worker(QtCore.QRunnable):
    def __init__(self, fn, *args, **kwargs):
        QtCore.QRunnable.__init__(self)
        self.fn = fn
        self.args = args
        self.kwargs = kwargs

    def run(self) -> None:
        self.fn(*self.args, **self.kwargs)

and self.ImageViewer.showImages refers to the below section of the ImageViewer class:

    def loadImg(self, file):
        tabName = str()
        if len(file) > 15: tabName = file[0:13] + "..."

        widget = ImageViewer()
        localPath = os.path.abspath(file)
        localPath = QtCore.QDir().filePath(localPath)
        pixmap = QtGui.QPixmap(localPath)
        widget.setImage(pixmap)
        self.tabwidget.addTab(widget, self.mainFormHandle.sharedData.imgIcon, tabName)
        self.tabwidget.setCurrentIndex(self.tabwidget.currentIndex() + 1)

    def **showImages**(self, files):
        files = [file.lower() for file in files]
        for file in files:
            self.loadImg(file)

Nothing much happens when I run this code. It just freezes and after a while the application closes with exit code -1073740791 (0xC0000409). What do you suggest to be the cause?

Answer 1

The issue seems to be that you are trying to create widgets from another thread than the main thread. You can only create widgets from the main thread but since loadImg() creates several widgets, calling it from the thread pool crashes your application. One way to get around this is to split loadImg() into two methods, one method that loads the pixmaps, and another one that creates the ImageViewers and adds the tabs. The first one can then be moved to the thread pool, and you can use a signal to automatically call the second one every time a pixmap is loaded. For example:

class TabArea(QtWidgets.QWidget):
    pixmap_loaded = QtCore.pyqtSignal(str, QtGui.QPixmap)

    def __init__(self, parent=None):
        ...
        self.pixmap_loaded.connect(self.addTab)

    def loadImg(self, file):
        localPath = os.path.abspath(file)
        localPath = QtCore.QDir().filePath(localPath)
        pixmap = QtGui.QPixmap(localPath)
        # emit pixmap and file name
        self.pixmap_loaded.emit(file, pixmap)

    def addTab(self, file, pixmap):
        tabName = str()
        if len(file) > 15: tabName = file[0:13] + "..."
        widget = ImageViewer()
        widget.setImage(pixmap)
        self.tabwidget.addTab(widget, self.mainFormHandle.sharedData.imgIcon, tabName)
        self.tabwidget.setCurrentIndex(self.tabwidget.currentIndex() + 1)

    def showImages(self, files):
        files = [file.lower() for file in files]
        for file in files:
            self.loadImg(file)