Question

I’m trying to learn how to use threading in a python program. I’m using PySide and QThreads since I’m going to implement gui afterwards with PySide.

I have understood the main consept of threading, at least I think. But I’m still confused with event loops. And I think that is the problem with my aplication.

Here is a sample application that I can’t get to work properly.
In my main class I have several worker threads and I want to them to report their progress to the main main class. But the main program don’t print progress messages in real time.

How could I get this to work?

from PySide import QtCore
import time, sys

class MyWorkerThread(QtCore.QThread):
    message = QtCore.Signal(str)

    def __init__(self, id, parent=None):
        super(MyWorkerThread, self).__init__(parent)
        self.id = id

    def run(self):
        for i in range(10):
            self.message.emit("%d: %d" % (self.id, i))
            time.sleep(0.2)

class MainProgram():
    def __init__(self, parent=None):
        self.threads = []

        self.addWorker(MyWorkerThread(1))
        self.addWorker(MyWorkerThread(2))

    def addWorker(self, worker):
        worker.message.connect(self.printMessage, QtCore.Qt.QueuedConnection)
        self.threads.append(worker)

    def startWorkers(self):
        for worker in self.threads:
            worker.start()
            worker.wait()
        self.workersFinished()

    def workersFinished(self):
        QtCore.QCoreApplication.instance().quit()

    @QtCore.Slot(str)
    def printMessage(self, text):
        sys.stdout.write(text+'\n')
        sys.stdout.flush()

if __name__ == '__main__':
    app = QtCore.QCoreApplication(sys.argv)
    m = MainProgram()
    m.startWorkers()
    sys.exit(app.exec_())

Answer 1

worker.wait() is the problem. This call blocks the main thread (in this case the one running event loop) until the worker finishes its job.

Here is a slightly changed version (I’ve commented my changes):

from PySide import QtCore
import time, sys

class MyWorkerThread(QtCore.QThread):
    message = QtCore.Signal(str)

    def __init__(self, id, parent=None):
        super(MyWorkerThread, self).__init__(parent)
        self.id = id

    def run(self):
        for i in range(10):
            self.message.emit("%d: %d" % (self.id, i))
            time.sleep(0.2)

class MainProgram():
    def __init__(self, parent=None):
        self.threads = []

        self.addWorker(MyWorkerThread(1))
        self.addWorker(MyWorkerThread(2))

    def addWorker(self, worker):
        worker.message.connect(self.printMessage, QtCore.Qt.QueuedConnection)
        # connect the finished signal to method so that we are notified
        worker.finished.connect(self.workersFinished)
        self.threads.append(worker)

    def startWorkers(self):
        for worker in self.threads:
            worker.start()
            # no wait, no finished. you start the threads and leave.

    def workersFinished(self):
        if all(worker.isFinished() for worker in self.threads):
            # wait until all the threads finished
            QtCore.QCoreApplication.instance().quit()

    @QtCore.Slot(str)
    def printMessage(self, text):
        sys.stdout.write(text+'\n')
        sys.stdout.flush()

if __name__ == '__main__':
    app = QtCore.QCoreApplication(sys.argv)
    m = MainProgram()
    m.startWorkers()
    sys.exit(app.exec_())