PyQt QThread MultiThreading does not work

Question

I have 2 QListWidget lists, List2 is being filled when some item has been selected from List1 QListWidgets

The problem is that, before filling List2 i have to do alot of tasks which freezes my UI for about 5 seconds which is too annoying, i want to make it fill List2 with QThread but it's not working since before initilizing whole class I'm getting an annoying error

from ui import Ui_Win
from PyQt4 import QtGui, QtCore

class GenericThread(QtCore.QThread):
    def __init__(self, parent=None):
        QtCore.QThread.__init__(self, parent)

    def __del__(self):
        self.quit()
        self.wait()

    def run(self):
        self.emit( QtCore.SIGNAL('itemSelectionChanged()'))
        return

class MainUI(QtGui.QMainWindow):
    def __init__(self, parent=None):
        QtGui.QMainWindow.__init__(self)
        self.ui = Ui_Win()
        self.ui.setupUi(self)
        ...

        genericThread = GenericThread(self)
        self.connect(genericThread, QtCore.SIGNAL("itemSelectionChanged()"), self.fill_List2 )
        genericThread.start()

    def fill_List2(self):
        self.ui.List2.clear()
        list1SelectedItem = str(self.ui.List1.currentItem().text()) # ERROR HERE

Traceback: # AttributeError: 'NoneType' object has no attribute 'text'

This accures because self.ui.List1.currentItem().text() is None

Why this fucntion is being called before triggering itemSelectionChanged signal?

It isn't. You start the thread, which immediately emits the `itemSelectionChanged` signal, which then calls `fill_List2`. What did you expect to happen? — ekhumoro, May 19 '15 at 16:56
@ekhumoro i thought thread should start when i click on an item from List1...how do i have to fix the code to make it work as expected? — PYPL, May 19 '15 at 19:16
You need to connect the `itemSelectionChanged` of `List1` to a slot which creates the thread, and starts it. Once the thread has finished, it should emit a custom signal which sends back the items for `List2` (you must not attempt to directly update the gui in the worker thread). — ekhumoro, May 19 '15 at 21:47
What is `from ui import Ui_Win`? I'm not familiar with that import and therefore it makes question extremely hard to answer. You also haven't shown the code to set up or add the list widgets. I will write up an answer, but the question would be more useful for future users if you could show what `ui` is and also add the setup code. — J Richard Snape, May 20 '15 at 14:10

PYPL · Accepted Answer · 2015-06-23T07:19:10.213

3

from ui import Ui_Win ## ui.py is a file that has been generated from Qt Designer and it contains main GUI objects like QListWidget
from PyQt4 import QtGui, QtCore

class GenericThread(QtCore.QThread):
    def __init__(self, parent=None, listIndex=0):
        QtCore.QThread.__init__(self, parent)
        self.listIndex = listIndex

    def __del__(self):
        self.quit()
        self.wait()

    def run(self):
        if self.listIndex == 2:
            for addStr in Something:
                #Some long stuff
                self.emit( QtCore.SIGNAL('fillListWithItems(QString, int'), addStr, self.listIndex)

class MainUI(QtGui.QMainWindow):
    def __init__(self, parent=None):
        QtGui.QMainWindow.__init__(self)
        self.ui = Ui_Win()
        self.ui.setupUi(self)
        ...

        self.ui.List1.list1SelectedItem.connect(self.fill_List2)

    @QtCore.pyqtSlot(QString, int)
    def self.fillListWithItems(addStr, lstIdx):
        if lstIdx==2:
            self.ui.List2.addItem(addStr)

    def fill_List2(self):
        self.ui.List2.clear()
        list1SelectedItem = str(self.ui.List1.currentItem().text())

        genericThread = GenericThread(self, listIndex=2)
        self.connect(genericThread, QtCore.SIGNAL("fillListWithItems(QString, int)"), self.fillListWithItems )
        genericThread.start()

Thanks @ekhumoro

edited Jun 23 '15 at 07:19

answered May 20 '15 at 13:48

PYPL

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Ah, I see you've got a working version for yourself. I came to this via the comment you left in Python chat a while ago - don't know why it didn't show me that you'd already got the answer. Anyhow - I'll repeat my comment here - I think it would be helpful for any future visitors if you outlined what `ui` was and showed how `List1` and `List2` were populated – J Richard Snape May 20 '15 at 14:18
And your solution with the custom signal is better - I'm not sure mine is very good even though it works, so I think I'll delete it or edit it. – J Richard Snape May 20 '15 at 14:20
@JRichardSnape And the `Ui_Win` is nothing but generated code from QtDesigner, it doesn't make any major sense – PYPL May 20 '15 at 14:51
OK - I'll undelete it, I guess it makes an alternative and maybe someone will comment if they don't like it emitting an itemSelectionChanged. Understood on the `UI_Win` front - maybe just put a line in stating that's what it is and that it doesn't contain anything very useful, but does hold the `List1` and `List2` widgets and they are `QListWidget` type (I assume they are) – J Richard Snape May 20 '15 at 14:57

score 0 · Answer 2 · answered May 20 '15 at 14:15

Your problem is that simply starting the thread is firing the itemSelectionChanged() signal (because it is in the run function), which you have connected to your fill_List2() function. You need to connect the itemSelectionChanged event on List1 to a SLOT that will trigger the thread to do the heavy computation and update List2.

I've had to make a number of assumptions about what ui is and what List1 and List2 are and how they're set up, but here is a working example. I've replaced the heavy computation in your GenericThread with a simple 2 second delay.

If I've misinterpreted / made bad assumptions, please update the question and leave a comment

test_slotting.py

from ui import Ui_Win
from PyQt4 import QtGui, QtCore
from PyQt4.QtCore import pyqtSlot
from PyQt4.QtGui import *
import time

class GenericThread(QtCore.QThread):
    def __init__(self, parent=None):
        QtCore.QThread.__init__(self, parent)

    def __del__(self):
        self.quit()
        self.wait()

    def run(self):
        #Do all your heavy processing here
        #I'll just wait for 2 seconds
        time.sleep(2)
        self.emit( QtCore.SIGNAL('itemSelectionChanged()'))
        return

class MainUI(QtGui.QMainWindow):
    def __init__(self, parent=None):
        QtGui.QMainWindow.__init__(self)
        self.ui = Ui_Win()
        self.ui.setupUi(self)
        self.ui.List1 = QListWidget(self)
        self.ui.List2 = QListWidget(self)

        hbox = QtGui.QHBoxLayout()
        hbox.addStretch(1)
        hbox.addWidget(self.ui.List1)
        hbox.addWidget(self.ui.List2)

        self.ui.centralWidget.setLayout(hbox)

        self.ui.List1.addItems(['alpha','beta','gamma','delta','epsilon'])
        self.ui.List2.addItems(['Item1','Item2'])

        self.ui.List1.itemSelectionChanged.connect(self.start_heavy_processing_thread)

    @pyqtSlot()
    def start_heavy_processing_thread(self):
        genericThread = GenericThread(self)
        self.connect(genericThread, QtCore.SIGNAL("itemSelectionChanged()"), self.fill_List2 )
        genericThread.start()

    def fill_List2(self):
        self.ui.List2.clear()
        list1SelectedItem = str(self.ui.List1.currentItem().text())
        self.ui.List2.addItem(list1SelectedItem)

if __name__ == "__main__":
    import sys
    app = QtGui.QApplication(sys.argv)
    MainWindow = MainUI()
    MainWindow.show()
    sys.exit(app.exec_())

ui.py

from PyQt4 import QtCore, QtGui

class Ui_Win(object):
    def setupUi(self, MainWindow):
        MainWindow.setObjectName("MainWindow")
        MainWindow.resize(416, 292)
        self.centralWidget = QtGui.QWidget(MainWindow)
        self.centralWidget.setObjectName("centralWidget")
        MainWindow.setCentralWidget(self.centralWidget)

if __name__ == "__main__":
    import sys
    app = QtGui.QApplication(sys.argv)
    MainWindow = QtGui.QMainWindow()
    ui = Ui_Win()
    ui.setupUi(MainWindow)
    MainWindow.show()
    sys.exit(app.exec_())

PyQt QThread MultiThreading does not work

2 Answers2

test_slotting.py

ui.py

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