{"id":5348,"date":"2022-10-21T03:57:05","date_gmt":"2022-10-21T03:57:05","guid":{"rendered":"https:\/\/www.pythontutorial.net\/?page_id=5348"},"modified":"2022-11-09T01:53:46","modified_gmt":"2022-11-09T01:53:46","slug":"pyqt-qthread","status":"publish","type":"page","link":"https:\/\/www.pythontutorial.net\/pyqt\/pyqt-qthread\/","title":{"rendered":"PyQt QThread"},"content":{"rendered":"\n

Summary<\/strong>: in this tutorial, you’ll learn how to use the PyQt QThread<\/code> to create a responsive Qt application.<\/p>\n\n\n\n

Introduction to the PyQt QThread class #<\/a><\/h2>\n\n\n\n

If a program has long-running operations, it may lag for a short moment. In some cases, the program is frozen completely.<\/p>\n\n\n\n

Therefore, when developing PyQt programs, you should know how to handle these situations. And to do that, you can take the advantage of threading.<\/p>\n\n\n\n

If you’re not familiar with the threading<\/a> concept, you can learn more about it in the Python concurrency<\/a> series.<\/p>\n\n\n\n

Python has a number of modules for handling threads such as threading and concurrent.futures<\/code>. <\/p>\n\n\n\n

While you can use these modules, PyQt provides a better way of doing it by using the QThread<\/code> class and other classes.<\/p>\n\n\n\n

The main thread and worker threads #<\/a><\/h3>\n\n\n\n

Qt applications are event-based. When you call the exec()<\/code> method, it starts an event loop and creates a thread that is referred to as the main thread<\/strong>.<\/p>\n\n\n\n

\"PyQt<\/figure>\n\n\n\n

Any events that occur in the main thread run synchronously within the main event loop. <\/p>\n\n\n\n

To take the advantage of threading, you need to create a secondary thread<\/strong> to offload the long-running operations from the main thread. The secondary threads are often referred to as worker threads<\/strong>.<\/p>\n\n\n\n

To communicate between the main thread and the worker threads, you use signals and slots. The steps for using the QThread<\/code> class are as follows:<\/p>\n\n\n\n

First, create a class that inherits from the QObject<\/code> and offloads the long-running operations to this class.<\/p>\n\n\n

class<\/span> Worker<\/span>(QObject)<\/span>:<\/span>\n   pass<\/span><\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
The reason we subclass the QObject<\/code> class is that we want to use the signal and slot.<\/p>\n\n\n\n
Next, create a worker thread and worker object from the main thread<\/p>\n\n\n
self.worker = Worker()\nself.worker_thread = QThread()<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
The self is an instance of the QMainWindow<\/code> or QWidget<\/code>.<\/p>\n\n\n\n
Then, connect signals and slots of the Worker class with the main thread.<\/p>\n\n\n\n
After that, move the worker to the worker thread by calling the moveToThread()<\/code> method of the worker object:<\/p>\n\n\n
self.worker.moveToThread(self.worker_thread)<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
Finally, start the worker thread:<\/p>\n\n\n
self.worker_thread.start()<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
It’s important to note that you should only communicate with the worker via signals and slots. And you do not call any of its methods from the main thread. For example:<\/p>\n\n\n
self.worker.do_some_work() # DON'T<\/span><\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
PyQt QThread example #<\/a><\/h2>\n\n\n\n
We’ll create a simple program that uses the QThread<\/code>:<\/p>\n\n\n\n
\"PyQt<\/figure>\n\n\n\n
The program consists of a progress bar<\/a> and a button<\/a>. When you click the start button, the long-running operation will run in a worker thread and update the progress<\/a> back to the main thread via signals and slots<\/a>.<\/p>\n\n\n\n
Here’s the complete program:<\/p>\n\n\n
import<\/span> sys\nfrom<\/span> PyQt6.QtWidgets import<\/span> QApplication, QMainWindow, QWidget, QLabel, QPushButton, QVBoxLayout, QProgressBar\nfrom<\/span> PyQt6.QtCore import<\/span> QThread, QObject, pyqtSignal as<\/span> Signal, pyqtSlot as<\/span> Slot\nimport<\/span> time\n\n\nclass<\/span> Worker<\/span>(QObject)<\/span>:<\/span>\n    progress = Signal(int)\n    completed = Signal(int)\n\n    @Slot(int)<\/span>\n    def<\/span> do_work<\/span>(self, n)<\/span>:<\/span>\n        for<\/span> i in<\/span> range(1<\/span>, n+1<\/span>):\n            time.sleep(1<\/span>)\n            self.progress.emit(i)\n\n        self.completed.emit(i)\n\n\nclass<\/span> MainWindow<\/span>(QMainWindow)<\/span>:<\/span>\n    work_requested = Signal(int)\n\n    def<\/span> __init__<\/span>(self, *args, **kwargs)<\/span>:<\/span>\n        super().__init__(*args, **kwargs)\n\n        self.setGeometry(100<\/span>, 100<\/span>, 300<\/span>, 50<\/span>)\n        self.setWindowTitle('QThread Demo'<\/span>)\n\n        # setup widget<\/span>\n        self.widget = QWidget()\n        layout = QVBoxLayout()\n        self.widget.setLayout(layout)\n        self.setCentralWidget(self.widget)       \n\n        self.progress_bar = QProgressBar(self)\n        self.progress_bar.setValue(0<\/span>)\n\n        self.btn_start = QPushButton('Start'<\/span>, clicked=self.start)\n\n        layout.addWidget(self.progress_bar)\n        layout.addWidget(self.btn_start)\n\n        self.worker = Worker()\n        self.worker_thread = QThread()\n\n        self.worker.progress.connect(self.update_progress)\n        self.worker.completed.connect(self.complete)\n\n        self.work_requested.connect(self.worker.do_work)\n\n        # move worker to the worker thread<\/span>\n        self.worker.moveToThread(self.worker_thread)\n\n        # start the thread<\/span>\n        self.worker_thread.start()\n\n        # show the window<\/span>\n        self.show()\n\n    def<\/span> start<\/span>(self)<\/span>:<\/span>\n        self.btn_start.setEnabled(False<\/span>)\n        n = 5<\/span>\n        self.progress_bar.setMaximum(n)\n        self.work_requested.emit(n)\n\n    def<\/span> update_progress<\/span>(self, v)<\/span>:<\/span>\n        self.progress_bar.setValue(v)\n\n    def<\/span> complete<\/span>(self, v)<\/span>:<\/span>\n        self.progress_bar.setValue(v)\n        self.btn_start.setEnabled(True<\/span>)\n\n\nif<\/span> __name__ == '__main__'<\/span>:\n    app = QApplication(sys.argv)\n    window = MainWindow()\n    sys.exit(app.exec())<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
How it works.<\/p>\n\n\n\n
Defining the worker class #<\/a><\/h3>\n\n\n\n
The Worker<\/code> class inherits from QObject<\/code> class so that it can support signals and slots. In practice, you move the long operations to the Worker class:<\/p>\n\n\n
class<\/span> Worker<\/span>(QObject)<\/span>:<\/span><\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
The Worker<\/code> class has two signals:<\/p>\n\n\n\n
Note that another way of using the QThread<\/code> class is to subclass it and override the run()<\/code> method. However, it is not recommended way. Please find the detailed answer here<\/a>.<\/p>\n\n\n\n