QThread

The QThread class provides a platform-independent way to manage threads. More…

Inheritance diagram of PySide2.QtCore.QThread

Synopsis

Functions

Virtual functions

Slots

Static functions

Detailed Description

A QThread object manages one thread of control within the program. QThreads begin executing in run() . By default, 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 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 QThread and reimplement 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 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. 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 QThread .

Unlike queued slots or invoked methods, methods called directly on the QThread object will execute in the thread that calls the method. When subclassing QThread , keep in mind that the constructor executes in the old thread while 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. As a general rule, functions can only be called from the thread that created the QThread object itself (e.g. setPriority() ), unless the documentation says otherwise. See Synchronizing Threads for details.

Managing Threads

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

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

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

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

Note

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

The static functions currentThreadId() and currentThread() return identifiers for the currently executing thread. The former returns a platform specific ID for the thread; the latter returns a 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 setObjectName() before starting the thread. If you don’t call 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 QThread subclass). Note that this is currently not available with release builds on Windows.

See also

QThreadStorage Mandelbrot Example Semaphores Example Wait Conditions Example

class PySide2.QtCore.QThread([parent=None])
param parent:

PySide2.QtCore.QObject

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

See also

start()

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 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 None .

See also

setEventDispatcher()

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

int

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

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

Note

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

See also

quit() exit()

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

retcode – 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 exec() . The 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 exec() has been called again. If the eventloop in exec() is not running then the next call to exec() will also return immediately.

See also

quit() QEventLoop

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

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:

bool

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

See also

isRunning()

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

bool

Return true if the task running on this thread should be stopped. An interruption can be requested by 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;
        }
    }
}

Note

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

See also

currentThread() requestInterruption()

PySide2.QtCore.QThread.isRunning()
Return type:

bool

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

See also

isFinished()

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

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 – int

Forces the current thread to sleep for msecs milliseconds.

Avoid using this function if you need to wait for a given condition to change. Instead, connect a slot to the signal that indicates the change or use an event handler (see event() ).

Note

This function does not guarantee accuracy. The application may sleep longer than msecs under heavy load conditions. Some OSes might round msecs up to 10 ms or 15 ms.

See also

sleep() usleep()

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

Priority

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

See also

Priority setPriority() start()

PySide2.QtCore.QThread.quit()

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

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

See also

exit() QEventLoop

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.

See also

isInterruptionRequested()

PySide2.QtCore.QThread.run()

The starting point for the thread. After calling 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.

See also

start() wait()

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

eventDispatcher – PySide2.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 start() or, in case of the main thread, before QCoreApplication has been instantiated. This method takes ownership of the object.

See also

eventDispatcher()

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

priority – Priority

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

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

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).

See also

Priority priority() start()

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

stackSize – 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.

See also

stackSize()

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

enabled – bool

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

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

See also

terminate()

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

arg__1 – int

Forces the current thread to sleep for secs seconds.

Avoid using this function if you need to wait for a given condition to change. Instead, connect a slot to the signal that indicates the change or use an event handler (see event() ).

Note

This function does not guarantee accuracy. The application may sleep longer than secs under heavy load conditions.

See also

msleep() usleep()

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

uint

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

See also

setStackSize()

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

priority – Priority

Begins execution of the thread by calling 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).

See also

run() terminate()

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 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 setTerminationEnabled() . Calling this function while termination is disabled results in the termination being deferred, until termination is re-enabled. See the documentation of setTerminationEnabled() for more information.

See also

setTerminationEnabled()

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

arg__1 – int

Forces the current thread to sleep for usecs microseconds.

Avoid using this function if you need to wait for a given condition to change. Instead, connect a slot to the signal that indicates the change or use an event handler (see event() ).

Note

This function does not guarantee accuracy. The application may sleep longer than usecs under heavy load conditions. Some OSes might round usecs up to 10 ms or 15 ms; on Windows, it will be rounded up to a multiple of 1 ms.

See also

sleep() msleep()

PySide2.QtCore.QThread.wait([deadline=QDeadlineTimer(QDeadlineTimer.Forever)])
Parameters:

deadline – PySide2.QtCore.QDeadlineTimer

Return type:

bool

Blocks the thread until either of these conditions is met:

  • The thread associated with this QThread object has finished execution (i.e. when it returns from run() ). This function will return true if the thread has finished. It also returns true if the thread has not been started yet.

  • The deadline is reached. This function will return false if the deadline is reached.

A deadline timer set to QDeadlineTimer::Forever (the default) will never time out: in this case, the function only returns when the thread returns from run() or if the thread has not yet started.

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

See also

sleep() terminate()

PySide2.QtCore.QThread.wait(time)
Parameters:

time – int

Return type:

bool

This is an overloaded 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.