Simple example of the correct way to use (Py)Qt(5) and QThread

threads1.py

# Copyright (c) 2016 Rob Kent (jazzycamel)
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# furnished to do so, subject to the following conditions:
#
# The above copyright notice and this permission notice shall be included in all copies or 
# substantial portions of the Software.
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# NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND 
# NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, 
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# OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
from PyQt5.QtCore import *
from PyQt5.QtWidgets import *
from itertools import count, islice

class Threaded(QObject):
    result=pyqtSignal(int)

    def __init__(self, parent=None, **kwargs):
        super().__init__(parent, **kwargs)

    @pyqtSlot()
    def start(self): print("Thread started")

    @pyqtSlot(int)
    def calculatePrime(self, n):
        primes=(n for n in count(2) if all(n % d for d in range(2, n)))
        self.result.emit(list(islice(primes, 0, n))[-1])

class GUI(QWidget):
    requestPrime=pyqtSignal(int)

    def __init__(self, parent=None, **kwargs):
        super().__init__(parent, **kwargs)

        self._thread=QThread()
        self._threaded=Threaded(result=self.displayPrime)
        self.requestPrime.connect(self._threaded.calculatePrime)
        self._thread.started.connect(self._threaded.start)
        self._threaded.moveToThread(self._thread)
        qApp.aboutToQuit.connect(self._thread.quit)
        self._thread.start()

        l=QVBoxLayout(self)
        self._iterationLE=QLineEdit(self, placeholderText="Iteration (n)")
        l.addWidget(self._iterationLE)
        self._requestBtn=QPushButton(
            "Calculate Prime", self, clicked=self.primeRequested)
        l.addWidget(self._requestBtn)
        self._busy=QProgressBar(self)
        l.addWidget(self._busy)
        self._resultLbl=QLabel("Result:", self)
        l.addWidget(self._resultLbl)

    @pyqtSlot()
    def primeRequested(self):
        try: n=int(self._iterationLE.text())
        except: return
        self.requestPrime.emit(n)
        self._busy.setRange(0,0)
        self._iterationLE.setEnabled(False)
        self._requestBtn.setEnabled(False)

    @pyqtSlot(int)
    def displayPrime(self, prime):
        self._resultLbl.setText("Result: {}".format(prime))
        self._busy.setRange(0,100)
        self._iterationLE.setEnabled(True)
        self._requestBtn.setEnabled(True)

if __name__=="__main__":
    from sys import exit, argv

    a=QApplication(argv)
    g=GUI()
    g.show()
    exit(a.exec_())

Its the equivalent of self._threaded.result.connect(self.displayPrime), i.e. you pass a reference to the instance of GUI.displayPrime.

I don't have a hyperlink anywhere, but you can basically pass the name of any objects signal to its constructor as a keyword argument with a reference to an instance of the target object's slot.

Fun fact: If using PySide2 instead of PyQt5, start() is called on the UI thread instead of the worker thread.

Workaround: Use this thread class instead of the original:

class QThread2(QThread):
	started2 = Signal()

	def __init__(self):
		QThread.__init__(self)
		self.started.connect(self.onStarted)

	def onStarted(self):
		self.started2.emit()

Fun fact: If using PySide2 instead of PyQt5, start() is called on the UI thread instead of the worker thread.

Workaround: Use this thread class instead of the original:

class QThread2(QThread):
	started2 = Signal()

	def __init__(self):
		QThread.__init__(self)
		self.started.connect(self.onStarted)

	def onStarted(self):
		self.started2.emit()

@serg06 Works fine in PySide2 for me (without your 'workaround') with the following import adaptations for PySide2:

from PySide2.QtCore import *
from PySide2.QtWidgets import *

pyqtSignal=Signal
pyqtSlot=Slot

from itertools import count, islice

class Threaded(QObject):
    result=pyqtSignal(int)

    def __init__(self, parent=None, **kwargs):
        super().__init__(parent, **kwargs)

    @pyqtSlot()
    def start(self): print("Thread started")

    @pyqtSlot(int)
    def calculatePrime(self, n):
        primes=(n for n in count(2) if all(n % d for d in range(2, n)))
        self.result.emit(list(islice(primes, 0, n))[-1])

class GUI(QWidget):
    requestPrime=pyqtSignal(int)

    def __init__(self, parent=None, **kwargs):
        super().__init__(parent, **kwargs)

        self._thread=QThread()
        self._threaded=Threaded()
        self._threaded.result.connect(self.displayPrime)
        self.requestPrime.connect(self._threaded.calculatePrime)
        self._thread.started.connect(self._threaded.start)
        self._threaded.moveToThread(self._thread)
        qApp.aboutToQuit.connect(self._thread.quit)
        self._thread.start()

        l=QVBoxLayout(self)
        self._iterationLE=QLineEdit(self, placeholderText="Iteration (n)")
        l.addWidget(self._iterationLE)
        self._requestBtn=QPushButton(
            "Calculate Prime", self, clicked=self.primeRequested)
        l.addWidget(self._requestBtn)
        self._busy=QProgressBar(self)
        l.addWidget(self._busy)
        self._resultLbl=QLabel("Result:", self)
        l.addWidget(self._resultLbl)

    @pyqtSlot()
    def primeRequested(self):
        try: n=int(self._iterationLE.text())
        except: return
        self.requestPrime.emit(n)
        self._busy.setRange(0,0)
        self._iterationLE.setEnabled(False)
        self._requestBtn.setEnabled(False)

    @pyqtSlot(int)
    def displayPrime(self, prime):
        self._resultLbl.setText("Result: {}".format(prime))
        self._busy.setRange(0,100)
        self._iterationLE.setEnabled(True)
        self._requestBtn.setEnabled(True)

if __name__=="__main__":
    from sys import exit, argv

    a=QApplication(argv)
    g=GUI()
    g.show()
    exit(a.exec_())

I tested this using Python 3.8.5 and PySide5.15.2 on Ubuntu 20.04.2LTS. What versions/OS are you running and how did you determine which thread code was running in?

Did you test my code or your own code using this method? There are a couple of gotchas with QThread; for example, if the object you move to the thread with QThread.moveToThread has a parent, then it will not actually get moved, but will not fail either.

Hi @jazzycamel

I would like to use the contents of this git in pyqtgraph's documentation with citation. Would you be so kind to put a license to your example(s) so I can properly cite?

Thank you so much for posting these.

@j9ac9k I've added the MIT license as a header to the original gist code. Hope this is what you need, glad this has been useful and thanks very much for the citation, much appreciated.

@j9ac9k I've added the MIT license as a header to the original gist code. Hope this is what you need, glad this has been useful and thanks very much for the citation, much appreciated.

That's perfect, thank you so much!

And it's in our docs: https://pyqtgraph.readthedocs.io/en/latest/getting_started/qtcrashcourse.html#id8

Thanks!