How to use QThread correctly in pyqt with moveToThread()?

Question

i read this article How To Really, Truly Use QThreads; The Full Explanation, it says instead of subclass qthread, and reimplement run(), one should use moveToThread to push a QObject onto QThread instance using moveToThread(QThread*)

here is the c++ example, but i don't know how to convert it to python code.

class Worker : public QObject
 {
     Q_OBJECT
     QThread workerThread;

 public slots:
     void doWork(const QString &parameter) {
         // ...
         emit resultReady(result);
     }

 signals:
     void resultReady(const QString &result);
 };

 class Controller : public QObject
 {
     Q_OBJECT
     QThread workerThread;
 public:
     Controller() {
         Worker *worker = new Worker;
         worker->moveToThread(&workerThread);
         connect(workerThread, SIGNAL(finished()), worker, SLOT(deleteLater()));
         connect(this, SIGNAL(operate(QString)), worker, SLOT(doWork(QString)));
         connect(worker, SIGNAL(resultReady(QString)), this, SLOT(handleResults(QString)));
         workerThread.start();
     }
     ~Controller() {
         workerThread.quit();
         workerThread.wait();
     }
 public slots:
     void handleResults(const QString &);
 signals:
     void operate(const QString &);
 };



QThread* thread = new QThread;
Worker* worker = new Worker();
worker->moveToThread(thread);
connect(worker, SIGNAL(error(QString)), this, SLOT(errorString(QString)));
connect(thread, SIGNAL(started()), worker, SLOT(process()));
connect(worker, SIGNAL(finished()), thread, SLOT(quit()));
connect(worker, SIGNAL(finished()), worker, SLOT(deleteLater()));
connect(thread, SIGNAL(finished()), thread, SLOT(deleteLater()));
thread->start();

i've been using this method to generate a qthread , but as you can see, it's using the not recommended way. how can i re-write it to use the preferred method ?

class GenericThread(QThread):
    def __init__(self, function, *args, **kwargs):
        QThread.__init__(self)
        # super(GenericThread, self).__init__()

        self.function = function
        self.args = args
        self.kwargs = kwargs

    def __del__(self):
        self.wait()

    def run(self, *args):
        self.function(*self.args, **self.kwargs)

edit: two years later ... I tried qris' code, it works and in different thread

import sys
import time
from PyQt4 import QtCore, QtGui
from PyQt4.QtCore import pyqtSignal, pyqtSlot
import threading


def logthread(caller):
    print('%-25s: %s, %s,' % (caller, threading.current_thread().name,
                              threading.current_thread().ident))


class MyApp(QtGui.QWidget):

    def __init__(self, parent=None):
        QtGui.QWidget.__init__(self, parent)

        self.setGeometry(300, 300, 280, 600)
        self.setWindowTitle('using threads')

        self.layout = QtGui.QVBoxLayout(self)

        self.testButton = QtGui.QPushButton("QThread")
        self.testButton.released.connect(self.test)
        self.listwidget = QtGui.QListWidget(self)

        self.layout.addWidget(self.testButton)
        self.layout.addWidget(self.listwidget)

        self.threadPool = []
        logthread('mainwin.__init__')

    def add(self, text):
        """ Add item to list widget """
        logthread('mainwin.add')
        self.listwidget.addItem(text)
        self.listwidget.sortItems()

    def addBatch(self, text="test", iters=6, delay=0.3):
        """ Add several items to list widget """
        logthread('mainwin.addBatch')
        for i in range(iters):
            time.sleep(delay)  # artificial time delay
            self.add(text+" "+str(i))

    def test(self):
        my_thread = QtCore.QThread()
        my_thread.start()

        # This causes my_worker.run() to eventually execute in my_thread:
        my_worker = GenericWorker(self.addBatch)
        my_worker.moveToThread(my_thread)
        my_worker.start.emit("hello")
        # my_worker.finished.connect(self.xxx)

        self.threadPool.append(my_thread)
        self.my_worker = my_worker


class GenericWorker(QtCore.QObject):

    start = pyqtSignal(str)
    finished = pyqtSignal()

    def __init__(self, function, *args, **kwargs):
        super(GenericWorker, self).__init__()
        logthread('GenericWorker.__init__')
        self.function = function
        self.args = args
        self.kwargs = kwargs
        self.start.connect(self.run)

    @pyqtSlot()
    def run(self, *args, **kwargs):
        logthread('GenericWorker.run')
        self.function(*self.args, **self.kwargs)
        self.finished.emit()


# run
app = QtGui.QApplication(sys.argv)
test = MyApp()
test.show()
app.exec_()

the ouput is:

mainwin.__init__         : MainThread, 140221684574016,
GenericWorker.__init__   : MainThread, 140221684574016,
GenericWorker.run        : Dummy-1, 140221265458944,
mainwin.addBatch         : Dummy-1, 140221265458944,
mainwin.add              : Dummy-1, 140221265458944,
mainwin.add              : Dummy-1, 140221265458944,
mainwin.add              : Dummy-1, 140221265458944,
mainwin.add              : Dummy-1, 140221265458944,
mainwin.add              : Dummy-1, 140221265458944,
mainwin.add              : Dummy-1, 140221265458944,

Answer 1

The default run() implementation in QThread runs an event loop for you, the equivalent of:

class GenericThread(QThread):
    def run(self, *args):
        self.exec_()

The important thing about an event loop is that it allows objects owned by the thread to receive events on their slots, which will be executed in that thread. Those objects are just QObjects, not QThreads.

Important note: the QThread object is not owned by its own thread [docs]:

It is important to remember that a QThread instance lives in the old thread that instantiated it, not in the new thread that calls run(). This means that all of QThread's queued slots and invoked methods will execute in the old thread [e.g. the main thread].

So you should be able to do this:

class GenericWorker(QObject):
    def __init__(self, function, *args, **kwargs):
        super(GenericWorker, self).__init__()

        self.function = function
        self.args = args
        self.kwargs = kwargs
        self.start.connect(self.run)

    start = pyqtSignal(str)

    @pyqtSlot()
    def run(self, some_string_arg):
        self.function(*self.args, **self.kwargs)

my_thread = QThread()
my_thread.start()

# This causes my_worker.run() to eventually execute in my_thread:
my_worker = GenericWorker(...)
my_worker.moveToThread(my_thread)
my_worker.start.emit("hello")

Also, think carefully about what happens with the result of self.function, which is currently discarded. You could declare another signal on GenericWorker, which receives the result, and have the run() method emit that signal when it's done, passing the result to it.

Once you get the hang of it and realize you don't and shouldn't subclass QThread, life becomes a lot more straightforward and easier. Simply put, never do work in QThread. You should almost never need to override run. For most use cases, setting up proper associations with a QObject to a QThread and using QT's signals/slots creates an extremely powerful way to do multithreaded programming. Just be careful not to let the QObjects you've pushed to your worker threads hang around...

http://ilearnstuff.blogspot.co.uk/2012/09/qthread-best-practices-when-qthread.html