Python PyQT: How to call a GUI function from a worker thread?

Question

I have a pyqt gui and calling a long process (ffmpeg) which I put on a separate thread to not block the gui. I then want to update a progress bar when one command of a longe

Answer 1

In short: Move to QThread and use Qt's signals and slots, they are the preferred way to communicate between threads.

This answer provides some examples how this could look like: https://stackoverflow.com/a/6789205/2319400

In your case, using the "SomeObject" version from the above could look like this:

class Worker(QtCore.QObject):

    madeProgress = QtCore.pyqtSignal([int])
    finished = QtCore.pyqtSignal()

    def __init__(self, cmdlist):
        self.cmdlist = cmdlist

    def run(self):
        for icmd, cmd in enumerate(self.cmdlist):
            # execute your work
            # processCommand(cmd)

            # signal that we've made progress
            self.madeProgress.emit(icmd)

        # emit the finished signal - we're done
        self.finished.emit()

Then move this worker to a QThread instance you create. Following the pattern from the linked answer, you can then connect the madeProgress signal to the setValue slot of a progressbar:

workerThread = QThread()
workerObject = Worker(cmdlist)
workerObject.moveToThread(workerThread)
workerThread.started.connect(workerObject.run)
workerObject.finished.connect(workerThread.quit)

# create a progressbar with min/max according to
# the length of your cmdlist
progressBar = QProgressBar()
progressBar.setRange(0, len(cmdlist))

# connect the worker's progress signal with the progressbar
workerObject.madeProgress.connect(progressBar.setValue)

# start the thread (starting your worker at the same time)
workerThread.start()