QThreadPool¶
The QThreadPool
class manages a collection of QThreads. More…

Synopsis¶
Functions¶
def
activeThreadCount
()def
clear
()def
contains
(thread)def
expiryTimeout
()def
maxThreadCount
()def
releaseThread
()def
reserveThread
()def
setExpiryTimeout
(expiryTimeout)def
setMaxThreadCount
(maxThreadCount)def
setStackSize
(stackSize)def
stackSize
()def
start
(runnable[, priority=0])def
tryStart
(runnable)def
tryTake
(runnable)def
waitForDone
([msecs=-1])
Static functions¶
def
globalInstance
()
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
-
class
PySide6.QtCore.
QThreadPool
([parent=None])¶ - Parameters
parent –
PySide6.QtCore.QObject
Constructs a thread pool with the given parent
.
-
PySide6.QtCore.QThreadPool.
activeThreadCount
()¶ - Return type
int
-
PySide6.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
-
PySide6.QtCore.QThreadPool.
contains
(thread)¶ - Parameters
thread –
PySide6.QtCore.QThread
- Return type
bool
Returns true
if thread
is a thread managed by this thread pool.
-
PySide6.QtCore.QThreadPool.
expiryTimeout
()¶ - Return type
int
See also
-
static
PySide6.QtCore.QThreadPool.
globalInstance
()¶ - Return type
Returns the global QThreadPool
instance.
-
PySide6.QtCore.QThreadPool.
maxThreadCount
()¶ - Return type
int
See also
-
PySide6.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
-
PySide6.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
-
PySide6.QtCore.QThreadPool.
setExpiryTimeout
(expiryTimeout)¶ - Parameters
expiryTimeout – int
See also
-
PySide6.QtCore.QThreadPool.
setMaxThreadCount
(maxThreadCount)¶ - Parameters
maxThreadCount – int
See also
-
PySide6.QtCore.QThreadPool.
setStackSize
(stackSize)¶ - Parameters
stackSize –
uint
See also
-
PySide6.QtCore.QThreadPool.
stackSize
()¶ - Return type
uint
See also
-
PySide6.QtCore.QThreadPool.
start
(runnable[, priority=0])¶ - Parameters
runnable –
PySide6.QtCore.QRunnable
priority – int
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.
-
PySide6.QtCore.QThreadPool.
tryStart
(runnable)¶ - Parameters
runnable –
PySide6.QtCore.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 on success 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.
-
PySide6.QtCore.QThreadPool.
tryTake
(runnable)¶ - Parameters
runnable –
PySide6.QtCore.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
-
PySide6.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).
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