PySide6.QtCore.QThread

class QThread

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

Inheritance diagram of PySide6.QtCore.QThread

Synopsis

Methods

Virtual methods

Slots

Signals

Static functions

Note

This documentation may contain snippets that were automatically translated from C++ to Python. We always welcome contributions to the snippet translation. If you see an issue with the translation, you can also let us know by creating a ticket on https:/bugreports.qt.io/projects/PYSIDE

Detailed Description

Warning

This section contains snippets that were automatically translated from C++ to Python and may contain errors.

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(QObject):

    Q_OBJECT
# public slots
    def doWork(parameter):
        result = QString()
        /* ... here is the expensive or blocking operation ... */
        resultReady.emit(result)

# signals
    def resultReady(result):

class Controller(QObject):

    Q_OBJECT
    workerThread = QThread()
# public
    Controller() {
        worker = Worker()
        worker.moveToThread(workerThread)
        workerThread.finished.connect(worker.deleteLater)
        self.operate.connect(worker.doWork)
        worker.resultReady.connect(self.handleResults)
        workerThread.start()

    ~Controller() {
        workerThread.quit()
        workerThread.wait()

# public slots
    def handleResults():
# signals
    def operate():

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(QThread):

    Q_OBJECT
    def run():
        result = QString()
        /* ... here is the expensive or blocking operation ... */
        resultReady.emit(result)

# signals
    def resultReady(s):

def startWorkInAThread(self):

    workerThread = WorkerThread(self)
    workerThread.resultReady.connect(self.handleResults)
    workerThread.finished.connect(workerThread.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.

You often want to deallocate objects that live in a thread when a thread ends. To do this, connect 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.

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 QChronoTimer .

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 Producer and Consumer using Semaphores Producer and Consumer using Wait Conditions

class 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 LowPriority.

QThread.LowPriority

scheduled less often than NormalPriority.

QThread.NormalPriority

the default priority of the operating system.

QThread.HighPriority

scheduled more often than NormalPriority.

QThread.HighestPriority

scheduled more often than HighPriority.

QThread.TimeCriticalPriority

scheduled as often as possible.

QThread.InheritPriority

use the same priority as the creating thread. This is the default.
__init__([parent=None])
Parameters:

parent – 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()

static currentThread()
Return type:

QThread

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

eventDispatcher()
Return type:

QAbstractEventDispatcher

Returns a pointer to the event dispatcher object for the thread. If no event dispatcher exists for the thread, this function returns None.

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()

exec_()
Return type:

int

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

finished()

This signal is emitted from the associated thread right before it finishes executing.

When this signal is emitted, the event loop has already stopped running. No more events will be processed in the thread, except for deferred deletion events. This signal can be connected to deleteLater() , to free objects in that thread.

Note

If the associated thread was terminated using terminate() , it is undefined from which thread this signal is emitted.

See also

started()

static idealThreadCount()
Return type:

int

Returns the ideal number of threads that this process can run in parallel. This is done by querying the number of logical processors available to this process (if supported by this OS) or the total number of logical processors in the system. This function returns 1 if neither value could be determined.

Note

On operating systems that support setting a thread’s affinity to a subset of all logical processors, the value returned by this function may change between threads and over time.

Note

On operating systems that support CPU hotplugging and hot-unplugging, the value returned by this function may also change over time (and note that CPUs can be turned on and off by software, without a physical, hardware change).

isCurrentThread()
Return type:

bool

Returns true if this thread is currentThread .

See also

currentThreadId()

isFinished()
Return type:

bool

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

See also

isRunning()

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()

static isMainThread()
Return type:

bool

Returns whether the currently executing thread is the main thread.

The main thread is the thread in which QCoreApplication was created. This is usually the thread that called the main() function, but not necessarily so. It is the thread that is processing the GUI events and in which graphical objects (QWindow, QWidget) can be created.

isRunning()
Return type:

bool

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

See also

isFinished()

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 msleep(msecs)
Parameters:

msecs – int

This is an overloaded function, equivalent to calling:

QThread::sleep(std::chrono::milliseconds{msecs});

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()

priority()
Return type:

Priority

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

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

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.

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()

setEventDispatcher(eventDispatcher)
Parameters:

eventDispatcher – 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.

An event dispatcher is automatically created for the main thread when QCoreApplication is instantiated and on start() for auxiliary threads.

This method takes ownership of the object.

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).

setStackSize(stackSize)
Parameters:

stackSize – int

Sets the stack size for the thread to stackSize. If stackSize is zero, the operating system or runtime will choose a default value. Otherwise, the thread’s stack size will be the value provided (which may be rounded up or down).

On most operating systems, the amount of memory allocated to serve the stack will initially be smaller than stackSize and will grow as the thread uses the stack. This parameter sets the maximum size it will be allowed to grow to (that is, it sets the size of the virtual memory space the stack is allowed to occupy).

This function can only be called before the thread is started.

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 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 sleep(secs)
Parameters:

secs – int

Forces the current thread to sleep for secs seconds.

This is an overloaded function, equivalent to calling:

QThread::sleep(std::chrono::seconds{secs});

See also

msleep() usleep()

stackSize()
Return type:

int

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

See also

setStackSize()

start([priority=QThread.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()

started()

This signal is emitted from the associated thread when it starts executing, so any slots connected to it may be called via queued invocation. Whilst the event may have been posted before run() is called, any cross-thread delivery of the signal may still be pending.

See also

run() finished()

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 terminate(), 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.

static usleep(usecs)
Parameters:

usecs – int

This is an overloaded function, equivalent to calling:

QThread::sleep(std::chrono::microseconds{secs});

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()

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

deadline – 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()

wait(time)
Parameters:

time – int

Return type:

bool

This is an overloaded function.

time is the time to wait in milliseconds. If time is ULONG_MAX, then the wait will never timeout.

static yieldCurrentThread()

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