How to use QMutex correctly with QThread?

Question

from PyQt5.QtCore import *
from PyQt5.QtWidgets import *


import time
import sys
import numpy as np

       

class Mainthread(QThread):

    def __init__(self, parent):
        super().__init__(parent)
        self.parent = parent
        self.running = None
        self.mutex = QMutex()



    def run(self):
                   
        while self.running:
            self.mutex.lock()
            print ("test")
            time.sleep(1)

            self.mutex.unlock()
                                                

class MainWindow(QMainWindow):

    def __init__(self):
        super().__init__()


        self.mainthread = Mainthread(self)
        self.mainthread.running = True
        self.mainthread.start()

        self.mainthread1 = Mainthread(self)
        self.mainthread1.running = True
        self.mainthread1.start()



    
app = QApplication(sys.argv)
mainwindow = MainWindow()
mainwindow.show()
app.exec_()

I have this code where I run two instances of the same MainThread class.

What I was expecting was that mainthread's message (which is "test") would print, then wait for a sec and then mainthread1's would be printed. Instead, it seems like both threads are running at the same time. Is there something I'm missing?

Answer 1

In your code, each thread creates its own separate mutex, so no relation is enforced between the two. Create a single mutex first, and pass it to the theads:

import time
import sys

from PyQt5.QtCore import QThread, QMutex
from PyQt5.QtWidgets import QMainWindow, QApplication


class Mainthread(QThread):
    def __init__(self, mutex, parent):
        super().__init__(parent)
        self.parent = parent
        self.running = None
        self.mutex = mutex

    def run(self):  
        while self.running:
            self.mutex.lock()
            print ("test")
            time.sleep(1)

            self.mutex.unlock()


class MainWindow(QMainWindow):
    def __init__(self):
        super().__init__()

        mutex = QMutex()

        self.mainthread = Mainthread(mutex, self)
        self.mainthread.running = True
        self.mainthread.start()

        self.mainthread1 = Mainthread(mutex, self)
        self.mainthread1.running = True
        self.mainthread1.start()


app = QApplication(sys.argv)
mainwindow = MainWindow()
mainwindow.show()
app.exec_()

Note: I don't have PyQt5 installed (and doing so on my architecture is tricky), but I tested this in PySide6 and as far as I know the behavior should be consistent.