Single worker thread for all tasks or multiple specific workers?

Question

I'm creating a simple GUI application using PyQt5 where I request some data from an API which is then used to populate various controls of the UI.

The examples I was following about worker threads in PyQt all seem to sub-class QThread and then do their business logic in the overridden run() method. This works fine but I want to execute different API calls at different times using a worker.

So my question is: do I need to create a specific worker thread for every operation I wish to do or is there a way of having a single thread class that I can use to carry out different operations at different times and therefore avoid the overhead of creating different thread sub-classes?

Answer 1

What you can do is design an object to do all these tasks (inherit QObject for slots / signals). Lets say each task is defined as a separate function - lets designate these functions as slots.

Then (a general order of events):

• instantiate a QThread object.
• instantiate your class.
• Move your object into the thread using YouClass->moveToThread(pThread).
• Now define a signal for each slot and connect these signals to the relevant slots in your object.
• Finally run the thread using pThread->start()

Now you can emit a signal to do a particular task in the thread. You do not need to sub-class QThread just use a normal class derived from QObject (so that you can use slots/signals).

You can either use one class in one thread to do many operations (note: they will be queued). Or make many classes in many threads (to run "parallel").

I don't know python well enough to attempt an example here so I won't :o

Note: The reason to sub-class QThread would be if you wanted to extend the functionality of the QThread class - i.e. add more/specific thread-related functions. QThread is a class that controls a thread, and is not meant to be used to run arbitrary/generic tasks... even though you can abuse it to do so if you wish :)

Answer 2

Here is a concise example of one (but could be as many as you wish) worker object which is moved to a single running QThread (started) and is communicating via signals. Also the thread is stopped at the end. It demonstrates what code_fodder outlined in his answer.

from PyQt4 import QtCore
QtCore.Signal = QtCore.pyqtSignal

class Master(QtCore.QObject):

    command = QtCore.Signal(str)

    def __init__(self):
        super().__init__()

class Worker(QtCore.QObject):

    def __init__(self):
        super().__init__()

    def do_something(self, text):
        print('current thread id = {}, message to worker = {}'.format(int(QtCore.QThread.currentThreadId()), text))

if __name__ == '__main__':

    app = QtCore.QCoreApplication([])

    # give us a thread and start it
    thread = QtCore.QThread()
    thread.start()

    # create a worker and move it to our extra thread
    worker = Worker()
    worker.moveToThread(thread)

    # create a master object and connect it to the worker
    master = Master()
    master.command.connect(worker.do_something)

    # call a method of the worker directly (will be executed in the actual thread)
    worker.do_something('wrong way to communicate with worker')

    # communicate via signals, will execute the method now in the extra thread
    master.command.emit('right way to communicate with worker')

    # start the application and kill it after 1 second
    QtCore.QTimer.singleShot(1000, app.quit)
    app.exec_()

    # don't forget to terminate the extra thread
    thread.quit()
    thread.wait(5000)