Details
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Bug
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Resolution: Incomplete
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P2: Important
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None
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5.11.1
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None
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Ubuntu 18.04 64-bit, Python3.6, PyQt5 5.11.2, PySide2 5.11.1
(P.S. This problem does not occur in Windows 10 64-bit with the same version of other packages and Python)
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Linux/X11
Description
I try to modify the original code written in PyQt5 to PySide2 as the license problems. In the original code (PyQt5), I use multiprocessing in QThread to increase CPU usage, and it works perfectly. However, after modified to PySide2, the code generates a series of error and destroy the GUI. The following is a minimized example to cause the problem.
The following code works well when I use the PyQt5 instead of PySide2.
import sys import multiprocessing as mp from PyQt5.QtCore import QThread from PyQt5.QtWidgets import QApplication, QMainWindow, QPushButton class Task(QThread): def run(self): print('task started') with mp.Pool() as pool: res = pool.map(mp_task, range(10000)) print('task finished', res) def mp_task(x): # some heavy tasks ret = 0 for i in range(x + 50000): ret += i return ret class Gui(QMainWindow): def __init__(self): super().__init__() button = QPushButton('click me') button.clicked.connect(self.do_task) self.setCentralWidget(button) def do_task(self): self.thread = Task() self.thread.start() def main(): app = QApplication(sys.argv) window = Gui() window.show() sys.exit(app.exec_()) if __name__ == '__main__': main()
However, if I change the import package from PyQt5 to PySide2, for example,
from PySide2.QtCore import QThread from PySide2.QtWidgets import QApplication, QMainWindow, QPushButton
After click the button, the GUI will be destroyed and the program generates the following error messages (some repeated messages has been omitted):
QObject: Cannot create children for a parent that is in a different thread. (Parent is QApplication(0x1cb19c0), parent's thread is QThread(0x1948cc0), current thread is Task(0x7fc1d0005390) QObject: Cannot create children for a parent that is in a different thread. (Parent is QApplication(0x1cb19c0), parent's thread is QThread(0x1948cc0), current thread is Task(0x7fc1d0005390) QObject: Cannot create children for a parent that is in a different thread. (Parent is QApplication(0x1cb19c0), parent's thread is QThread(0x1948cc0), current thread is Task(0x7fc1d0005390) qt.qpa.xcb: QXcbConnection: XCB error: 128 (Unknown), sequence: 548, resource id: 88080397, major code: 130 (Unknown), minor code: 2 qt.qpa.xcb: QXcbConnection: XCB error: 128 (Unknown), sequence: 552, resource id: 88080397, major code: 130 (Unknown), minor code: 2 QObject: Cannot create children for a parent that is in a different thread. (Parent is QApplication(0x1cb19c0), parent's thread is QThread(0x1948cc0), current thread is Task(0x7fc1d0005390)
Interestingly, if I put the contents of the main() function to global scope, the problem solved! For example:
if __name__ == '__main__': app = QApplication(sys.argv) window = Gui() window.show() sys.exit(app.exec_())
Maybe there are some differences between PyQt5 and PySide2 in QThread and lead to this fatal error. If I do not want to move QApplication and Gui into global scope, how could I fix the problem?