QThread

Inheritance diagram of PySide2.QtCore.QThread

Synopsis

Functions

Virtual functions

Slots

Static functions

Detailed Description

The PySide2.QtCore.QThread class provides a platform-independent way to manage threads.

A PySide2.QtCore.QThread object manages one thread of control within the program. QThreads begin executing in PySide2.QtCore.QThread.run() . By default, PySide2.QtCore.QThread.run() starts the event loop by calling exec() and runs a Qt event loop inside the thread.

You can use worker objects by moving them to the thread using QObject.moveToThread() .

class Worker : public QObject
{
    Q_OBJECT

public slots:
    void doWork(const QString &parameter) {
        QString result;
        /* ... here is the expensive or blocking operation ... */
        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, &QThread::finished, worker, &QObject::deleteLater);
        connect(this, &Controller::operate, worker, &Worker::doWork);
        connect(worker, &Worker::resultReady, this, &Controller::handleResults);
        workerThread.start();
    }
    ~Controller() {
        workerThread.quit();
        workerThread.wait();
    }
public slots:
    void handleResults(const QString &);
signals:
    void operate(const QString &);
};

The code inside the Worker’s slot would then execute in a separate thread. However, you are free to connect the Worker’s slots to any signal, from any object, in any thread. It is safe to connect signals and slots across different threads, thanks to a mechanism called queued connections .

Another way to make code run in a separate thread, is to subclass PySide2.QtCore.QThread and reimplement PySide2.QtCore.QThread.run() . For example:

class WorkerThread : public QThread
{
    Q_OBJECT
    void run() override {
        QString result;
        /* ... here is the expensive or blocking operation ... */
        emit resultReady(result);
    }
signals:
    void resultReady(const QString &s);
};

void MyObject::startWorkInAThread()
{
    WorkerThread *workerThread = new WorkerThread(this);
    connect(workerThread, &WorkerThread::resultReady, this, &MyObject::handleResults);
    connect(workerThread, &WorkerThread::finished, workerThread, &QObject::deleteLater);
    workerThread->start();
}

In that example, the thread will exit after the run function has returned. There will not be any event loop running in the thread unless you call exec() .

It is important to remember that a PySide2.QtCore.QThread instance lives in the old thread that instantiated it, not in the new thread that calls PySide2.QtCore.QThread.run() . This means that all of PySide2.QtCore.QThread ‘s queued slots will execute in the old thread. Thus, a developer who wishes to invoke slots in the new thread must use the worker-object approach; new slots should not be implemented directly into a subclassed PySide2.QtCore.QThread .

When subclassing PySide2.QtCore.QThread , keep in mind that the constructor executes in the old thread while PySide2.QtCore.QThread.run() executes in the new thread. If a member variable is accessed from both functions, then the variable is accessed from two different threads. Check that it is safe to do so.

Note

Care must be taken when interacting with objects across different threads. See Synchronizing Threads for details.

Managing Threads

PySide2.QtCore.QThread will notifiy you via a signal when the thread is started() and finished() , or you can use PySide2.QtCore.QThread.isFinished() and PySide2.QtCore.QThread.isRunning() to query the state of the thread.

You can stop the thread by calling PySide2.QtCore.QThread.exit() or PySide2.QtCore.QThread.quit() . In extreme cases, you may want to forcibly PySide2.QtCore.QThread.terminate() an executing thread. However, doing so is dangerous and discouraged. Please read the documentation for PySide2.QtCore.QThread.terminate() and PySide2.QtCore.QThread.setTerminationEnabled() for detailed information.

From Qt 4.8 onwards, it is possible to deallocate objects that live in a thread that has just ended, by connecting the finished() signal to QObject.deleteLater() .

Use PySide2.QtCore.QThread.wait() to block the calling thread, until the other thread has finished execution (or until a specified time has passed).

PySide2.QtCore.QThread also provides static, platform independent sleep functions: PySide2.QtCore.QThread.sleep() , PySide2.QtCore.QThread.msleep() , and PySide2.QtCore.QThread.usleep() allow full second, millisecond, and microsecond resolution respectively. These functions were made public in Qt 5.0.

Note

PySide2.QtCore.QThread.wait() and the PySide2.QtCore.QThread.sleep() functions should be unnecessary in general, since Qt is an event-driven framework. Instead of PySide2.QtCore.QThread.wait() , consider listening for the finished() signal. Instead of the PySide2.QtCore.QThread.sleep() functions, consider using PySide2.QtCore.QTimer .

The static functions PySide2.QtCore.QThread.currentThreadId() and PySide2.QtCore.QThread.currentThread() return identifiers for the currently executing thread. The former returns a platform specific ID for the thread; the latter returns a PySide2.QtCore.QThread pointer.

To choose the name that your thread will be given (as identified by the command ps -L on Linux, for example), you can call PySide2.QtCore.QObject.setObjectName() before starting the thread. If you don’t call PySide2.QtCore.QObject.setObjectName() , the name given to your thread will be the class name of the runtime type of your thread object (for example, "RenderThread" in the case of the Mandelbrot Example , as that is the name of the PySide2.QtCore.QThread subclass). Note that this is currently not available with release builds on Windows.

class PySide2.QtCore.QThread([parent=nullptr])
Parameters:parentPySide2.QtCore.QObject

Constructs a new PySide2.QtCore.QThread to manage a new thread. The parent takes ownership of the PySide2.QtCore.QThread . The thread does not begin executing until PySide2.QtCore.QThread.start() is called.

PySide2.QtCore.QThread.Priority

This enum type indicates how the operating system should schedule newly created threads.

Constant Description
QThread.IdlePriority scheduled only when no other threads are running.
QThread.LowestPriority scheduled less often than .
QThread.LowPriority scheduled less often than .
QThread.NormalPriority the default priority of the operating system.
QThread.HighPriority scheduled more often than .
QThread.HighestPriority scheduled more often than .
QThread.TimeCriticalPriority scheduled as often as possible.
QThread.InheritPriority use the same priority as the creating thread. This is the default.
static PySide2.QtCore.QThread.currentThread()
Return type:PySide2.QtCore.QThread

Returns a pointer to a PySide2.QtCore.QThread which manages the currently executing thread.

PySide2.QtCore.QThread.eventDispatcher()
Return type:PySide2.QtCore.QAbstractEventDispatcher

Returns a pointer to the event dispatcher object for the thread. If no event dispatcher exists for the thread, this function returns 0.

PySide2.QtCore.QThread.exec_()
Return type:PySide2.QtCore.int

Enters the event loop and waits until PySide2.QtCore.QThread.exit() is called, returning the value that was passed to PySide2.QtCore.QThread.exit() . The value returned is 0 if PySide2.QtCore.QThread.exit() is called via PySide2.QtCore.QThread.quit() .

This function is meant to be called from within PySide2.QtCore.QThread.run() . It is necessary to call this function to start event handling.

PySide2.QtCore.QThread.exit([retcode=0])
Parameters:retcodePySide2.QtCore.int

Tells the thread’s event loop to exit with a return code.

After calling this function, the thread leaves the event loop and returns from the call to QEventLoop.exec() . The QEventLoop.exec() function returns returnCode .

By convention, a returnCode of 0 means success, any non-zero value indicates an error.

Note that unlike the C library function of the same name, this function does return to the caller – it is event processing that stops.

No QEventLoops will be started anymore in this thread until QThread.exec() has been called again. If the eventloop in QThread.exec() is not running then the next call to QThread.exec() will also return immediately.

static PySide2.QtCore.QThread.idealThreadCount()
Return type:PySide2.QtCore.int

Returns the ideal number of threads that can be run on the system. This is done querying the number of processor cores, both real and logical, in the system. This function returns 1 if the number of processor cores could not be detected.

PySide2.QtCore.QThread.isFinished()
Return type:PySide2.QtCore.bool

Returns true if the thread is finished; otherwise returns false .

PySide2.QtCore.QThread.isInterruptionRequested()
Return type:PySide2.QtCore.bool

Return true if the task running on this thread should be stopped. An interruption can be requested by PySide2.QtCore.QThread.requestInterruption() .

This function can be used to make long running tasks cleanly interruptible. Never checking or acting on the value returned by this function is safe, however it is advisable do so regularly in long running functions. Take care not to call it too often, to keep the overhead low.

void long_task() {
     forever {
        if ( QThread::currentThread()->isInterruptionRequested() ) {
            return;
        }
    }
}
PySide2.QtCore.QThread.isRunning()
Return type:PySide2.QtCore.bool

Returns true if the thread is running; otherwise returns false .

PySide2.QtCore.QThread.loopLevel()
Return type:PySide2.QtCore.int

Returns the current event loop level for the thread.

Note

This can only be called within the thread itself, i.e. when it is the current thread.

static PySide2.QtCore.QThread.msleep(arg__1)
Parameters:arg__1 – long

Forces the current thread to sleep for msecs milliseconds.

PySide2.QtCore.QThread.priority()
Return type:PySide2.QtCore.QThread.Priority

Returns the priority for a running thread. If the thread is not running, this function returns InheritPriority .

PySide2.QtCore.QThread.quit()

Tells the thread’s event loop to exit with return code 0 (success). Equivalent to calling QThread.exit (0).

This function does nothing if the thread does not have an event loop.

PySide2.QtCore.QThread.requestInterruption()

Request the interruption of the thread. That request is advisory and it is up to code running on the thread to decide if and how it should act upon such request. This function does not stop any event loop running on the thread and does not terminate it in any way.

PySide2.QtCore.QThread.run()

The starting point for the thread. After calling PySide2.QtCore.QThread.start() , the newly created thread calls this function. The default implementation simply calls exec() .

You can reimplement this function to facilitate advanced thread management. Returning from this method will end the execution of the thread.

PySide2.QtCore.QThread.setEventDispatcher(eventDispatcher)
Parameters:eventDispatcherPySide2.QtCore.QAbstractEventDispatcher

Sets the event dispatcher for the thread to eventDispatcher . This is only possible as long as there is no event dispatcher installed for the thread yet. That is, before the thread has been started with PySide2.QtCore.QThread.start() or, in case of the main thread, before PySide2.QtCore.QCoreApplication has been instantiated. This method takes ownership of the object.

PySide2.QtCore.QThread.setPriority(priority)
Parameters:priorityPySide2.QtCore.QThread.Priority

This function sets the priority for a running thread. If the thread is not running, this function does nothing and returns immediately. Use PySide2.QtCore.QThread.start() to start a thread with a specific priority.

The priority argument can be any value in the QThread::Priority enum except for InheritPriorty .

The effect of the priority parameter is dependent on the operating system’s scheduling policy. In particular, the priority will be ignored on systems that do not support thread priorities (such as on Linux, see http://linux.die.net/man/2/sched_setscheduler for more details).

PySide2.QtCore.QThread.setStackSize(stackSize)
Parameters:stackSizePySide2.QtCore.uint

Sets the maximum stack size for the thread to stackSize . If stackSize is greater than zero, the maximum stack size is set to stackSize bytes, otherwise the maximum stack size is automatically determined by the operating system.

Warning

Most operating systems place minimum and maximum limits on thread stack sizes. The thread will fail to start if the stack size is outside these limits.

static PySide2.QtCore.QThread.setTerminationEnabled([enabled=true])
Parameters:enabledPySide2.QtCore.bool

Enables or disables termination of the current thread based on the enabled parameter. The thread must have been started by PySide2.QtCore.QThread .

When enabled is false, termination is disabled. Future calls to QThread.terminate() will return immediately without effect. Instead, the termination is deferred until termination is enabled.

When enabled is true, termination is enabled. Future calls to QThread.terminate() will terminate the thread normally. If termination has been deferred (i.e. QThread.terminate() was called with termination disabled), this function will terminate the calling thread immediately . Note that this function will not return in this case.

static PySide2.QtCore.QThread.sleep(arg__1)
Parameters:arg__1 – long

Forces the current thread to sleep for secs seconds.

PySide2.QtCore.QThread.stackSize()
Return type:PySide2.QtCore.uint

Returns the maximum stack size for the thread (if set with PySide2.QtCore.QThread.setStackSize() ); otherwise returns zero.

PySide2.QtCore.QThread.start([priority=InheritPriority])
Parameters:priorityPySide2.QtCore.QThread.Priority

Begins execution of the thread by calling PySide2.QtCore.QThread.run() . The operating system will schedule the thread according to the priority parameter. If the thread is already running, this function does nothing.

The effect of the priority parameter is dependent on the operating system’s scheduling policy. In particular, the priority will be ignored on systems that do not support thread priorities (such as on Linux, see the sched_setscheduler documentation for more details).

PySide2.QtCore.QThread.terminate()

Terminates the execution of the thread. The thread may or may not be terminated immediately, depending on the operating system’s scheduling policies. Use QThread.wait() after , to be sure.

When the thread is terminated, all threads waiting for the thread to finish will be woken up.

Warning

This function is dangerous and its use is discouraged. The thread can be terminated at any point in its code path. Threads can be terminated while modifying data. There is no chance for the thread to clean up after itself, unlock any held mutexes, etc. In short, use this function only if absolutely necessary.

Termination can be explicitly enabled or disabled by calling QThread.setTerminationEnabled() . Calling this function while termination is disabled results in the termination being deferred, until termination is re-enabled. See the documentation of QThread.setTerminationEnabled() for more information.

static PySide2.QtCore.QThread.usleep(arg__1)
Parameters:arg__1 – long

Forces the current thread to sleep for usecs microseconds.

PySide2.QtCore.QThread.wait([time=ULONG_MAX])
Parameters:time – long
Return type:PySide2.QtCore.bool

Blocks the thread until either of these conditions is met:

  • The thread associated with this PySide2.QtCore.QThread object has finished execution (i.e. when it returns from PySide2.QtCore.QThread.run() ). This function will return true if the thread has finished. It also returns true if the thread has not been started yet.
  • time milliseconds has elapsed. If time is ULONG_MAX (the default), then the wait will never timeout (the thread must return from PySide2.QtCore.QThread.run() ). This function will return false if the wait timed out.

This provides similar functionality to the POSIX pthread_join() function.

static PySide2.QtCore.QThread.yieldCurrentThread()

Yields execution of the current thread to another runnable thread, if any. Note that the operating system decides to which thread to switch.