Why do external functions cause the PyQt5 window to freeze?

Question

Here is some sample code that breaks:

import sys
import time
from PyQt5.QtWidgets import (QApplication, QDialog,
                             QProgressBar)

class Actions(QDialog):
    def __init__(self):
        super().__init__()
        self.initUI()

    def initUI(self):
        self.progress = QProgressBar(self)
        self.progress.setGeometry(0, 0, 300, 25)
        self.show()

        self.count = 0

        while self.count < 100:
            self.count += 1
            time.sleep(1) # Example external function
            self.progress.setValue(self.count)

if __name__ == "__main__":
    app = QApplication(sys.argv)
    window = Actions()
    sys.exit(app.exec_())

Running this will cause it to freeze and become unresponsive particularly in windows environments. Replacing the time.sleep function with any non-PyQt5 function will yield the same results.

From what I understand this has to do with the function not being called in a separate thread using QThread. I used this answer as a reference and came up with a partial solution.

import sys
import time

from PyQt5.QtCore import QThread
from PyQt5.QtWidgets import (QApplication, QDialog,
                             QProgressBar)

class External(QThread):

    def run(self):
        count = 0

        while count < 100:
            count += 1
            print(count)
            time.sleep(1)

class Actions(QDialog):
    def __init__(self):
        super().__init__()
        self.initUI()

    def initUI(self):
        self.progress = QProgressBar(self)
        self.progress.setGeometry(0, 0, 300, 25)
        self.show()

if __name__ == "__main__":
    app = QApplication(sys.argv)
    window = Actions()
    calc = External()
    calc.finished.connect(app.exit)
    calc.start()
    sys.exit(app.exec_())

This will run time.sleep in the background and keep the main window responsive. But, I don't know how to update the values using self.progress.setValue since it's not accessible in class External.

So far from what I know, I have to use signals to accomplish this. Most of the documentation out there is for PyQt4 making it harder to find a solution.

Another problem I am faced with is being able to start the External thread from within class Actions.

Answers to this problem will also serve as valuable documentation for PyQt5. Thanks in advance.

This answer shows how to wire an external thread to a progress bar via signals / slots. I think its applicable to qt5 http://stackoverflow.com/questions/9682376/progress-bar-with-pyqt — tdelaney, Mar 26 '17 at 03:58

score 1 · Accepted Answer · answered Mar 26 '17 at 04:08

You must use the signals to update the values.

import sys
import time

from PyQt5.QtCore import QThread, pyqtSignal
from PyQt5.QtWidgets import (QApplication, QDialog,
                             QProgressBar)

class External(QThread):
    countChanged = pyqtSignal(int)
    def run(self):
        count = 0

        while count < 100:
            count += 1
            self.countChanged.emit(count)
            print(count)
            time.sleep(1)

class Actions(QDialog):
    def __init__(self):
        super().__init__()
        self.initUI()

    def initUI(self):
        self.progress = QProgressBar(self)
        self.progress.setGeometry(0, 0, 300, 25)
        self.show()

    def onCountChanged(self, value):
        self.progress.setValue(value)

if __name__ == "__main__":
    app = QApplication(sys.argv)
    window = Actions()
    calc = External()
    calc.countChanged.connect(window.onCountChanged)
    calc.start()
    sys.exit(app.exec_())

score 0 · Answer 2 · answered Mar 26 '17 at 04:33

Here is a version that starts the thread from inside class Actions and uses a button:

import sys
import time

from PyQt5.QtCore import QThread, pyqtSignal
from PyQt5.QtWidgets import (QApplication, QDialog,
                             QProgressBar, QPushButton)

class External(QThread):

    countChanged = pyqtSignal(int)

    def run(self):
        count = 0

        while count < 100:
            count += 1
            time.sleep(1)
            print(count)
            self.countChanged.emit(count)

class Actions(QDialog):
    def __init__(self):
        super().__init__()
        self.initUI()

    def initUI(self):
        self.progress = QProgressBar(self)
        self.progress.setGeometry(0, 0, 300, 25)
        self.button = QPushButton('Start', self)
        self.button.move(0, 30)
        self.show()

        self.button.clicked.connect(self.onButtonClick)

    def onButtonClick(self):
        self.calc = External()
        self.calc.countChanged.connect(self.onCountChanged)
        self.calc.start()

    def onCountChanged(self, value):
        self.progress.setValue(value)

if __name__ == "__main__":
    app = QApplication(sys.argv)
    window = Actions()
    sys.exit(app.exec_())

Why do external functions cause the PyQt5 window to freeze?

2 Answers2