[FIXED] pyQt and threading application crash

Issue

I wrote simple program which has pyQt interface with 2 buttons (start and cancel). Start button runs some calculations in the background (by starting update function) and thanks to threading I can still use UI. But the application crashes after 10sec - 2 minutes. UI just dissapears, program shutdown.

when I use pythonw to run app without console thread crashes after ~25 sec but gui still works.

#!/usr/bin/python
import threading
import sys
from PyQt4 import QtGui, QtCore
import time
import os


class Class(QtGui.QWidget):

    def __init__(self):
        #Some init variables
        self.initUI()

    def initUI(self):
        #some UI
        self.show()

    def update(self,stop_event):
            while True and not stop_event.isSet():
                self.updateSpeed()
                self.updateDistance()
                self.printLogs()
                self.saveCSV()
                self.guiUpdate()
                time.sleep(1)

    #gui button function
    def initiate(self):
        self.stop_event = threading.Event()
        self.c_thread = threading.Thread(target = self.update, args=(self.stop_event,))
        self.c_thread.start()

    #Also gui button function
    def cancelTracking(self):
        self.stop_event.set()
        self.close()

def main():

    app = QtGui.QApplication(sys.argv)
    ex = Class()
    sys.exit(app.exec_())
    ex.update()

if __name__ == '__main__':
    main()

I dont know if I'm doing threading right. I found example like this on stack. I'm quite new to python and I'm using threading for the first time.

Solution

It is most likely due to calling a GUI function in your separate thread. PyQt GUI calls like setText() on a QLineEdit are not allowed from a thread. Anything that has PyQt painting outside of the main thread will not work. One way to get around this is to have your thread emit a signal to update the GUI when data is ready. The other way is to have a timer periodically checking for new data and updating the paintEvent after a certain time.

========== EDIT ==========

To Fix this issue I created a library named qt_thread_updater. https://github.com/justengel/qt_thread_updater This works by continuously running a QTimer. When you call call_latest the QTimer will run the function in the main thread.

from qt_thread_updater import get_updater

lbl = QtWidgets.QLabel('Value: 1')

counter = {'a': 1}

def run_thread():
    while True:
        text = 'Value: {}'.format(counter['a'])
        get_updater().call_latest(lbl.setText, text)
        counter['a'] += 1
        time.sleep(0.1)

th = threading.Thread(target=run_thread)
th.start()

========== END EDIT ==========

#!/usr/bin/python
import threading
import sys
from PyQt4 import QtGui, QtCore
import time
import os


class Class(QtGui.QWidget):

    display_update = QtCore.pyqtSignal() # ADDED

    def __init__(self):
        #Some init variables
        self.initUI()

    def initUI(self):
        #some UI
        self.display_update.connect(self.guiUpdate) # ADDED
        self.show()

    def update(self):
        while True and not self.stop_event.isSet():
            self.updateSpeed()
            self.updateDistance()
            self.printLogs()
            self.saveCSV()

            # self.guiUpdate()
            self.display_update.emit() # ADDED

            time.sleep(1)

    #gui button function
    def initiate(self):
        self.stop_event = threading.Event()
        self.c_thread = threading.Thread(target = self.update)
        self.c_thread.start()

    #Also gui button function
    def cancelTracking(self):
        self.stop_event.set()
        self.close()

def main():

    app = QtGui.QApplication(sys.argv)
    ex = Class()
    sys.exit(app.exec_())
    # ex.update() # - this does nothing

if __name__ == '__main__':
    main()

The other thing that could be happening is deadlock from two threads trying to access the same variable. I've read that this shouldn't be possible in python, but I have experienced it from the combination of PySide and other Python C extension libraries.

May also want to join the thread on close or use the QtGui.QApplication.aboutToQuit signal to join the thread before the program closes.

Answered By - justengel