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Keep PyQt UI Responsive With Threads

I have created a relatively complex PyQt program and am trying to implement threads so that when the program encounters a part of the program which is particularly CPU intensive, the GUI will remain refreshed and responsive throughout. Sadly though, I am having some difficulties with the threading.
I am using Python 2.7 for reasons that I don't believe to be relevant.
Anyway, the entire program runs within one class and calls upon a PyQt designer .ui file in order to display the actual GUI. When a particular button is pressed, in order to shred a file, it calls a function within that class that then starts a thread using the 'thread' module, yes, outdated, I know. The shredding function that is then called from this commences the shredding of the file. Throughout the shredding of the file, the actual shredding function interacts and adds bits to the GUI in order to keep the user up to date on what is happening.
During the execution of the function the GUI continues to be refreshed, however it does become a little laggy, I can cope with that. However, when that function is complete, instead of smoothly continuing and allowing the user to keep using the program, the program throws a complete hissy fit and simply just stops working and has to be closed.
Hopefully someone can assist me here. I would greatly appreciate as much detail as possible as I have been searching around for a way to cope with this for a good number of weeks now.
I am using PyQt4.

Here's a simple demo of threading in pyqt5. Qt has it's own threading class that works pretty well.
from PyQt5 import QtCore, QtGui, QtWidgets
from PyQt5.QtCore import pyqtSignal
import sys
import time
class TheBoss(QtWidgets.QWidget):
def __init__(self, parent=None):
super(TheBoss, self).__init__(parent)
self.resize(300,200)
self.VL = QtWidgets.QVBoxLayout(self)
self.label = QtWidgets.QLabel()
self.VL.addWidget(self.label)
self.logger = Logger()
self.logger.sec_signal.connect(self.label.setText)
self.logger.start()
def closeEvent(self,event):
self.logger.terminate()
class Logger(QtCore.QThread):
sec_signal = pyqtSignal(str)
def __init__(self, parent=None):
super(Logger, self).__init__(parent)
self.current_time = 0
self.go = True
def run(self):
#this is a special fxn that's called with the start() fxn
while self.go:
time.sleep(1)
self.sec_signal.emit(str(self.current_time))
self.current_time += 1
if __name__ == '__main__':
app = QtWidgets.QApplication(sys.argv)
app.setApplicationName("Thread Example")
window = TheBoss()
window.show()
sys.exit(app.exec_())

Qthread is still working when i close gui on python pyqt

my code has thread, but when i close the gui, it still works on background. how can i stop threads? is there something stop(), close()?
i dont use signal, slots? Must i use this?
from PyQt4 import QtGui, QtCore
import sys
import time
import threading
class Main(QtGui.QMainWindow):
def __init__(self, parent=None):
super(Main, self).__init__(parent)
self.kac_ders=QtGui.QComboBox()
self.bilgi_cek=QtGui.QPushButton("Save")
self.text=QtGui.QLineEdit()
self.widgetlayout=QtGui.QFormLayout()
self.widgetlar=QtGui.QWidget()
self.widgetlar.setLayout(self.widgetlayout)
self.bilgiler=QtGui.QTextBrowser()
self.bilgi_cek.clicked.connect(self.on_testLoop)
self.scrollArea = QtGui.QScrollArea()
self.scrollArea.setWidgetResizable(True)
self.scrollArea.setWidget(self.widgetlar)
self.analayout=QtGui.QVBoxLayout()
self.analayout.addWidget(self.text)
self.analayout.addWidget(self.bilgi_cek)
self.analayout.addWidget(self.bilgiler)
self.centralWidget=QtGui.QWidget()
self.centralWidget.setLayout(self.analayout)
self.setCentralWidget(self.centralWidget)
def on_testLoop(self):
self.c_thread=threading.Thread(target=self.kontenjan_ara)
self.c_thread.start()
def kontenjan_ara(self):
while(1):
self.bilgiler.append(self.text.text())
time.sleep(10)
app = QtGui.QApplication(sys.argv)
myWidget = Main()
myWidget.show()
app.exec_()

A few things:
You shouldn't be calling GUI code from outside the main thread. GUI elements are not thread-safe. self.kontenjan_ara updates and reads from GUI elements, it shouldn't be the target of your thread.
In almost all cases, you should use QThreads instead of python threads. They integrate nicely with the event and signal system in Qt.
If you just want to run something every few seconds, you can use a QTimer
def __init__(self, parent=None):
...
self.timer = QTimer(self)
self.timer.timeout.connect(self.kontenjan_ara)
self.timer.start(10000)
def kontenjan_ara(self):
self.bilgiler.append(self.text.text())
If your thread operations are more computationally complex you can create a worker thread and pass data between the worker thread and the main GUI thread using signals.
class Worker(QObject):
work_finished = QtCore.pyqtSignal(object)
#QtCore.pyqtSlot()
def do_work(self):
data = 'Text'
while True:
# Do something with data and pass back to main thread
data = data + 'text'
self.work_finished.emit(data)
time.sleep(10)
class MyWidget(QtGui.QWidget):
def __init__(self, ...)
...
self.worker = Worker()
self.thread = QtCore.QThread(self)
self.worker.work_finished.connect(self.on_finished)
self.worker.moveToThread(self.thread)
self.thread.started.connect(self.worker.do_work)
self.thread.start()
#QtCore.pyqtSlot(object)
def on_finished(self, data):
self.bilgiler.append(data)
...
Qt will automatically kill all the subthreads when the main thread exits the event loop.

I chose to rewrite a bit this answer, because I had failed to properly look at the problem's context. As the other answers and comments tell, you code lacks thread-safety.
The best way to fix this is to try to really think "in threads", to restrict yourself to only use objects living in the same thread, or functions that are known as "threadsafe".
Throwing in some signals and slots will help, but maybe you want to think back a bit to your original problem. In your current code, each time a button is pressed, a new thread in launched, that will, every 10 seconds, do 2 things :
- Read some text from self.text
- Append it to self.bilgiler
Both of these operations are non-threadsafe, and must be called from the thread that owns these objects (the main thread). You want to make the worker threads "schedule & wait" the read & append oeprations, instead of simply "executing" them.
I recommend using the other answer (the thread halting problem is automatically fixed by using proper QThreads that integrate well with Qt's event loop), which would make you use a cleaner approach, more integrated with Qt.
You may also want to rethink your problem, because maybe there is a simpler approach to your problem, for example : not spawning threads each time bilgi_cek is clicked, or using Queue objects so that your worker is completely agnostic of your GUI, and only interact with it using threadsafe objects.
Good luck, sorry if I caused any confusion. My original answer is still available here. I think it would be wise to mark the other answer as the valid answer for this question.

Handling GTK Objects on Threads in Python

i'm creating a application in Python, that uses GTK to build the UI, and i'm little confuse about handle GTK Objects on Threads, for example, the GtkProgressBar object.
Here is the context:
I try to do a download on a Main Thread, and i add a GObject.timeout_add to pulse the bar until the Download ends. But after the first pulse the UI froze.
Until there OK, the Thread froze until the download is complete, so any component will be updated. Solution: Create a new Thread.
I've created this new thread to make the Download, and other things. On this thread, i receive the progress bar to do the updates. But while the download is running and i add the GObject.timeout_add to pulse the bar, the UI froze again. New Solution: Create a third thread.
So my threads is looking lke this:
Main-Thread
'---- Thread 1
'------Thread 2
So, on Thread 1, i make another things and update the UI, while in Thread 2 i make the download and add the GObject.timeout_add and in there i can update the progress bar. And in Thread 1 i join the Thread 2
I handle the Gtk Objects using GObject.idle_add function.
But i'm very confuse about why the download and the update of the progress bar works well on the Thread 2 and not on Thread 1
Someone can explain to me why that's happen, or if i miss a something about handle GTK objects.
Thank you

If you block the Gtk main loop, the window and the widgets become unresponsive. That's why you cannot make the download in the main thread. Updating the Gtk.ProgressBar from the download thread could work depending on how you implemented it.
This is one way to download something and have a working ProgressBar:
from gi.repository import Gtk, GLib
import threading
import urllib.request
class Window(Gtk.Window):
def __init__(self):
super().__init__()
self.connect('delete-event', Gtk.main_quit)
self.progress = Gtk.ProgressBar()
self.add(self.progress)
self.show_all()
# Start download
self.thread = threading.Thread(target=self.download)
self.thread.start()
GLib.timeout_add_seconds(1, self.download_pulse)
def download(self):
urllib.request.urlretrieve('<link>')
def download_pulse(self):
if self.thread.is_alive():
self.progress.pulse()
return True
return False
win = Window()
Gtk.main()

How to stop qt app from freezing the main program?

For example:
#!/usr/bin/env python3
import sys
from PySide import QtCore, QtGui
class Dialog(QtGui.QDialog):
def __init__(self):
QtGui.QDialog.__init__(self)
button = QtGui.QPushButton("test")
layout = QtGui.QVBoxLayout()
layout.addWidget(button)
self.setLayout(layout)
app = QtGui.QApplication(sys.argv)
toast = Dialog()
toast.show()
app.exec_()
print("App freezes the main process!")
The last print() function will not be executed until you close the dialog.
I am working on a script that only uses qt for displaying some content that does not require user interaction, so I would prefer the gui code runs in background.

This is not possible. Qt documentation states:
Although QObject is reentrant, the GUI classes, notably QWidget and all its subclasses, are not reentrant. They can only be used from the main thread. As noted earlier, QCoreApplication::exec() must also be called from that thread.
(emphasis mine)
This answer suggests on the other hand that in reality this is not true :) However it seems that PySide sticks to the official version:
This can be verified by the following code sample:
import sys
import threading
from PySide import QtCore, QtGui
class Dialog(QtGui.QDialog):
def __init__(self):
QtGui.QDialog.__init__(self)
button = QtGui.QPushButton("test")
layout = QtGui.QVBoxLayout()
layout.addWidget(button)
self.setLayout(layout)
app = QtGui.QApplication(sys.argv)
toast = Dialog()
toast.show()
t = threading.Thread(target = lambda: app.exec_())
t.daemon = True
t.start()
print("App freezes the main process!")
input()
which produces the following output:
App freezes the main process!
QApplication::exec: Must be called from the main thread
(and a crash, on my machine). I have also verified the option with creating the app within the other thread - it works, but crashes on exit.
So the solution seems to let Qt have the main thread, and organize your processing in a separate thread. This shouldn't really be a problem: if you'll separate your concerns well it won't make a difference for your console-app part on which thread it's running.

I'm not sure if the PySide imposes any restrictions, but here's how it's done in C++:
Instantiate QApplication in a secondary thread.
Create your dialog in that same thread.
Call either QDialog::exec() OR {QApplication::exec() plus QDialog::show()} in that same thread.
Make sure that your secondary thread has fully shut down before you quit your app.
Yes, the Qt documentation currently says that only the main thread is allowed. However, there is nothing in the Qt source code that forbids creating QApplication in a secondary thread and then using GUI classes in that thread (for Windows and Linux). The documentation should be changed.
Mac OS X is different though -- the Cocoa framework only allows GUI operations in the main thread.

Cannot send posted events for objects in another thread

When I try to use one QDialog object from threads I get this error.
Here is the code I'm using:
import threading
import test_qdialog
from PyQt4 import QtGui, QtCore
class MyThread(threading.Thread):
def __init__(self, id, window, mutex):
self.id = id
self.window = window
self.mutex = mutex
super(MyThread, self).__init__()
def run(self):
with self.mutex:
result = self.window.exec_()
if result == QtGui.QDialog.Accepted:
print "Thread %d: %s" % (self.id, self.window.message_input.text())
if __name__ == "__main__":
import sys
app = QtGui.QApplication(sys.argv)
mutex = threading.Lock()
threads = []
window = test_qdialog.MyDialog()
for i in range(5):
thread = MyThread(i, window, mutex)
thread.start()
threads.append(thread)
for thread in threads:
thread.join()
sys.exit(app.exec_())
As written in this answer, if I get it right, I can't do it this way. But how can I do it then?

You can only create and use GUI widgets on main thread (every UI library that I know is like that). However, you can easily pass signals from threads to main using QtCore.QtThread. See for example the answer to PyQt threads and signals - how to properly retrieve values (even if the answer is not what the OP was looking for, it is relevant to your situation). May also find this SO post useful.
So instead of creating or accessing the dialog from thread, you would emit a signal from thread, and have your main window connected to it create the dialog when it receives the signal. Qt takes care of transfering data between threads. Will work like a charm.
Definitely take a close look at Qt Threading Basics, if you haven't already (if you have, may want to post questions about parts you don't understand, there is tons of important info there).

The QT Widgets can't be accessed from a thread which is not the main thread. For example, if you call mainwindow.show(), the program will crash.
However, this can be easily addressed using QThread. The principle is that instead of controlling the e.g., mainwindow directly, we can send a signal to the main thread, letting the main thread to call the show() method. A signal can carry anything, such as a String or Integer.
I'd strongly suggest you to watch this video. It will solve your problem in 10 minutes.