[PySide6] Slot Executed in Signal's Thread Even With Auto & QueuedConnection
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The correct way to implement threads is to overwrite QThread.run() to do the work. We also recommend using the @Slot decorator. Consider:
import sys from PySide6.QtCore import QObject, Signal, QThread, Qt, Slot from PySide6.QtWidgets import QMainWindow, QApplication class Worker(QThread): update = Signal() def run(self): print(f"worker: {QThread.currentThread().objectName()}") self.update.emit() class Window(QMainWindow): def __init__(self, parent=None): super().__init__(parent) self._worker = Worker() self._worker.setObjectName('test thread') self._worker.update.connect(self.update) self._worker.start() def closeEvent(self, e): self._worker.wait() e.accept() @Slot() def update(self): print(f"window: {QThread.currentThread().objectName()}") if __name__ == '__main__': app = QApplication() w = Window() w.show() sys.exit(app.exec())wrote on 21 Jan 2025, 19:02 last edited by@friedemannkleint said in [PySide6] Slot Executed in Signal's Thread Even With Auto & QueuedConnection:
The correct way to implement threads is to overwrite QThread.run() to do the work.
Why is there only one correct approach?!
Shouldn't the worker thread approach work the same (as also when using C++)?Here in this topic the worker approach seems to function correctly (besides the mistakes OP made there).
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@friedemannkleint said in [PySide6] Slot Executed in Signal's Thread Even With Auto & QueuedConnection:
The correct way to implement threads is to overwrite QThread.run() to do the work.
Why is there only one correct approach?!
Shouldn't the worker thread approach work the same (as also when using C++)?Here in this topic the worker approach seems to function correctly (besides the mistakes OP made there).
wrote on 21 Jan 2025, 20:17 last edited byThanks for the reply! I did read that post. I’m really confused to see that one works because it’s almost the same as mine. Do you have any idea why this is happening?
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wrote on 21 Jan 2025, 20:55 last edited by thinkinmachine
It's reported to Qt Bug Tracker: https://bugreports.qt.io/browse/PYSIDE-2990
Update: tested cpp equivalent version and it worked fine. It should be a PySide issue.
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wrote on 22 Jan 2025, 07:24 last edited by friedemannkleint
Please don't use that technique of connecting a worker to the QThread.start() signal. It leaks a thread hanging in its exec() function since that is called by the default implementation of run(), plus the signal execution is blocked by the work.
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T thinkinmachine has marked this topic as solved on 22 Jan 2025, 16:45
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Please don't use that technique of connecting a worker to the QThread.start() signal. It leaks a thread hanging in its exec() function since that is called by the default implementation of run(), plus the signal execution is blocked by the work.
wrote on 22 Jan 2025, 16:48 last edited by thinkinmachine@friedemannkleint
Thank you so much. I didn't noticeupdatewould be a issue.However I found another issue. When I use PyCharm's debugger to run the code the thread of worker becomes main thread again. It might have to do with debugger settings.
Update: When the main thread is blocked by breakpoint, the child threads are all blocked as well. This ONLY happens with PyCharm debugging.
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@friedemannkleint
Thank you so much. I didn't noticeupdatewould be a issue.However I found another issue. When I use PyCharm's debugger to run the code the thread of worker becomes main thread again. It might have to do with debugger settings.
Update: When the main thread is blocked by breakpoint, the child threads are all blocked as well. This ONLY happens with PyCharm debugging.
wrote on 22 Jan 2025, 23:35 last edited by FlowOverNestedThreads@thinkinmachine can confirm (dear lord, how have you even pinpointed the reason?), also related
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wrote on 23 Jan 2025, 06:13 last edited by thinkinmachine
Also, it doesn't work if
lambdais used. Probably because Qt doesn't know which thread the lambda function should be in.self._worker.update.connect(lambda: self.slotUpdate()) -
I'm having trouble with signal & slot in different threads. The slot is always executed in the signal caller's thread, even with explicit
QueuedConnection. Below is the minimal example.import sys from PySide6.QtCore import QObject, Signal, QThread, Qt from PySide6.QtWidgets import QMainWindow, QApplication class Worker(QObject): update = Signal() def run(self): print(f"worker: {QThread.currentThread().objectName()}") self.update.emit() class Window(QMainWindow): def __init__(self, parent=None): super().__init__(parent) worker = Worker() thread = QThread() thread.setObjectName('test thread') worker.moveToThread(thread) thread.started.connect(worker.run) worker.update.connect(self.update) thread.start() def update(self): print(f"window: {QThread.currentThread().objectName()}") if __name__ == '__main__': app = QApplication() w = Window() w.show() sys.exit(app.exec())The output of this minimal example is
worker: Qt mainThread window: test threadSeems like the slot is always executed in signal caller's thread, because the thread of
thread.startis main thread, and the thread ofworker.updateis test thread. The result is the same after I addQueuedConnectionexplicitly.I've read the document of
QThreadseveral times, including this one, and it seems like I'm using threads in the correct way. Please correct me if I'm wrong.My PySide version is 6.8.1. Any help would be greatly appreciated.
wrote on 23 Jan 2025, 12:13 last edited by@thinkinmachine said in [PySide6] Slot Executed in Signal's Thread Even With Auto & QueuedConnection:
Probably because Qt doesn't know
It's not a Qt thing I would say, rather than Python-related.
I'm not the best at Python, but AFAIK there is not "context" in Python lambdas (correct me if I'm wrong).
In C++ the Qt Signal-Slot lambda is executed in the "context" (if any) object's thread, or the receiving one.
Like// ------------------------------------ Worker w; int result = 42; emit w.workFinished(result); // ------------------------------------ connect(&w, &Worker::workFinished, this, [=](int res) { int result = res + 123; doSomethingWith(result); }); // ------------------------------------ -
@thinkinmachine can confirm (dear lord, how have you even pinpointed the reason?), also related
wrote on 24 Jan 2025, 00:10 last edited byI reported this bug to JetBrain PyCharm team (here). Hopefully they can help figure out what's going on. Thanks for providing stackoverflow link.
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@thinkinmachine said in [PySide6] Slot Executed in Signal's Thread Even With Auto & QueuedConnection:
Probably because Qt doesn't know
It's not a Qt thing I would say, rather than Python-related.
I'm not the best at Python, but AFAIK there is not "context" in Python lambdas (correct me if I'm wrong).
In C++ the Qt Signal-Slot lambda is executed in the "context" (if any) object's thread, or the receiving one.
Like// ------------------------------------ Worker w; int result = 42; emit w.workFinished(result); // ------------------------------------ connect(&w, &Worker::workFinished, this, [=](int res) { int result = res + 123; doSomethingWith(result); }); // ------------------------------------wrote on 24 Jan 2025, 00:11 last edited byThanks for your explanation! I'll keep you posted if I find out something new on this topic.
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