在使用 matplotlib 和 PySide2 运行的应用程序中使用 pdb 调试器时，“事件循环已经在运行”

Question

I have an application in which the user has to click on images using matplotlib. I want to use the debugger pdb (line 53), however, when I launch the app and click on the button, a message

QCoreApplication::exec: The event loop is already running

appears and prevents me of using the debugger. I suspect that it comes from the following lines but I'm not sure

import matplotlib
try:
    matplotlib.rcParams['backend.qt5'] = 'PySide2'
except (KeyError, Exception):
    pass
matplotlib.use('Qt5Agg')

I did implement those lines thanks to the answer I got from this question I asked previously: Same code with PyQT5/PySide2 runs on MacOS but throws an error on Linux

How could I retain the same code structure with backend and being able to use the pdb debugger?

import sys
import os 
import subprocess
from subprocess import call
import matplotlib
import pdb

try:
    matplotlib.rcParams['backend.qt5'] = 'PySide2'
except (KeyError, Exception):
    pass
matplotlib.use('Qt5Agg')
import matplotlib.pyplot as plt


from PySide2 import QtGui
from PySide2 import QtCore
from PySide2.QtWidgets import QWidget, QApplication, QPushButton, QFileDialog
from PySide2.QtCore import QFile, QTextStream

os.environ['QT_MAC_WANTS_LAYER'] = '1'


from IPython.core import ultratb
import pdb

sys.excepthook = ultratb.FormattedTB(mode='Verbose', color_scheme='Linux', call_pdb=True)


class GUI(QWidget):
    def __init__(self):
        super(GUI, self).__init__()

        self.initUI()

    def initUI(self):

        height_btn = 40 
        width_btn = 350
        
        button_position_x = 0
        button_position_y = 20 

        button_position_x = button_position_x = 0
        button_position_y = button_position_y + 400
        btn15 = QPushButton('button', self)     
        btn15.clicked.connect(self.Plotfunction)
        btn15.resize(width_btn, height_btn)
        btn15.move(button_position_y, button_position_x)       
        self.show()

    def Plotfunction(self):
        pdb.set_trace()
        print("ok")


def main():

    app = QApplication(sys.argv)
    ex = GUI()
    sys.exit(app.exec_())


if __name__ == '__main__':
    main()

Answer 1

By default matplotlib will load the binding (PyQt or PySide2) that is already imported, or if there are none loaded then it will try to import them and the first one that manages to import it will use it. Apparently in the case of the OP it is first importing PyQt causing that problem. The solution is to import PySide2 and then matplotlib so that it prefers PySide2 as the backend binding.