QThreadPool

The QThreadPool class manages a collection of QThreads. More…

Inheritance diagram of PySide2.QtCore.QThreadPool

Synopsis

Functions

Static functions

Detailed Description

QThreadPool manages and recyles individual QThread objects to help reduce thread creation costs in programs that use threads. Each Qt application has one global QThreadPool object, which can be accessed by calling globalInstance() .

To use one of the QThreadPool threads, subclass QRunnable and implement the run() virtual function. Then create an object of that class and pass it to start() .

class HelloWorldTask(QRunnable):
    def run(self):
        print "Hello world from thread", QThread.currentThread()

hello = HelloWorldTask()
# QThreadPool takes ownership and deletes 'hello' automatically
QThreadPool.globalInstance().start(hello)

QThreadPool deletes the QRunnable automatically by default. Use setAutoDelete() to change the auto-deletion flag.

QThreadPool supports executing the same QRunnable more than once by calling tryStart (this) from within run() . If autoDelete is enabled the QRunnable will be deleted when the last thread exits the run function. Calling start() multiple times with the same QRunnable when autoDelete is enabled creates a race condition and is not recommended.

Threads that are unused for a certain amount of time will expire. The default expiry timeout is 30000 milliseconds (30 seconds). This can be changed using setExpiryTimeout() . Setting a negative expiry timeout disables the expiry mechanism.

Call maxThreadCount() to query the maximum number of threads to be used. If needed, you can change the limit with setMaxThreadCount() . The default maxThreadCount() is idealThreadCount() . The activeThreadCount() function returns the number of threads currently doing work.

The reserveThread() function reserves a thread for external use. Use releaseThread() when your are done with the thread, so that it may be reused. Essentially, these functions temporarily increase or reduce the active thread count and are useful when implementing time-consuming operations that are not visible to the QThreadPool .

Note that QThreadPool is a low-level class for managing threads, see the Qt Concurrent module for higher level alternatives.

See also

QRunnable

class QThreadPool([parent=None])
param parent

QObject

Constructs a thread pool with the given parent .

PySide2.QtCore.QThreadPool.activeThreadCount()
Return type

int

PySide2.QtCore.QThreadPool.cancel(runnable)
Parameters

runnable – QRunnable

Removes the specified runnable from the queue if it is not yet started. The runnables for which runnable->autoDelete() returns true are deleted.

See also

start() tryTake()

PySide2.QtCore.QThreadPool.clear()

Removes the runnables that are not yet started from the queue. The runnables for which runnable->autoDelete() returns true are deleted.

See also

start()

PySide2.QtCore.QThreadPool.expiryTimeout()
Return type

int

See also

setExpiryTimeout()

static PySide2.QtCore.QThreadPool.globalInstance()
Return type

QThreadPool

Returns the global QThreadPool instance.

PySide2.QtCore.QThreadPool.maxThreadCount()
Return type

int

See also

setMaxThreadCount()

PySide2.QtCore.QThreadPool.releaseThread()

Releases a thread previously reserved by a call to reserveThread() .

Note

Calling this function without previously reserving a thread temporarily increases maxThreadCount() . This is useful when a thread goes to sleep waiting for more work, allowing other threads to continue. Be sure to call reserveThread() when done waiting, so that the thread pool can correctly maintain the activeThreadCount() .

See also

reserveThread()

PySide2.QtCore.QThreadPool.reserveThread()

Reserves one thread, disregarding activeThreadCount() and maxThreadCount() .

Once you are done with the thread, call releaseThread() to allow it to be reused.

Note

This function will always increase the number of active threads. This means that by using this function, it is possible for activeThreadCount() to return a value greater than maxThreadCount() .

See also

releaseThread()

PySide2.QtCore.QThreadPool.setExpiryTimeout(expiryTimeout)
Parameters

expiryTimeout – int

See also

expiryTimeout()

PySide2.QtCore.QThreadPool.setMaxThreadCount(maxThreadCount)
Parameters

maxThreadCount – int

See also

maxThreadCount()

PySide2.QtCore.QThreadPool.setStackSize(stackSize)
Parameters

stackSize – uint

See also

stackSize()

PySide2.QtCore.QThreadPool.stackSize()
Return type

uint

See also

setStackSize()

PySide2.QtCore.QThreadPool.start(runnable[, priority=0])
Parameters

Reserves a thread and uses it to run runnable , unless this thread will make the current thread count exceed maxThreadCount() . In that case, runnable is added to a run queue instead. The priority argument can be used to control the run queue’s order of execution.

Note that the thread pool takes ownership of the runnable if runnable->autoDelete() returns true , and the runnable will be deleted automatically by the thread pool after the runnable->run() returns. If runnable->autoDelete() returns false , ownership of runnable remains with the caller. Note that changing the auto-deletion on runnable after calling this functions results in undefined behavior.

PySide2.QtCore.QThreadPool.tryStart(runnable)
Parameters

runnable – QRunnable

Return type

bool

Attempts to reserve a thread to run runnable .

If no threads are available at the time of calling, then this function does nothing and returns false . Otherwise, runnable is run immediately using one available thread and this function returns true .

Note that the thread pool takes ownership of the runnable if runnable->autoDelete() returns true , and the runnable will be deleted automatically by the thread pool after the runnable->run() returns. If runnable->autoDelete() returns false , ownership of runnable remains with the caller. Note that changing the auto-deletion on runnable after calling this function results in undefined behavior.

PySide2.QtCore.QThreadPool.tryTake(runnable)
Parameters

runnable – QRunnable

Return type

bool

Attempts to remove the specified runnable from the queue if it is not yet started. If the runnable had not been started, returns true , and ownership of runnable is transferred to the caller (even when runnable->autoDelete() == true ). Otherwise returns false .

Note

If runnable->autoDelete() == true , this function may remove the wrong runnable. This is known as the ABA problem: the original runnable may already have executed and has since been deleted. The memory is re-used for another runnable, which then gets removed instead of the intended one. For this reason, we recommend calling this function only for runnables that are not auto-deleting.

See also

start() autoDelete()

PySide2.QtCore.QThreadPool.waitForDone([msecs=-1])
Parameters

msecs – int

Return type

bool

Waits up to msecs milliseconds for all threads to exit and removes all threads from the thread pool. Returns true if all threads were removed; otherwise it returns false . If msecs is -1 (the default), the timeout is ignored (waits for the last thread to exit).