{"id":4865,"date":"2022-09-16T06:46:52","date_gmt":"2022-09-16T06:46:52","guid":{"rendered":"https:\/\/www.pythontutorial.net\/?page_id=4865"},"modified":"2022-10-29T07:27:30","modified_gmt":"2022-10-29T07:27:30","slug":"pyqt-hello-world","status":"publish","type":"page","link":"https:\/\/www.pythontutorial.net\/pyqt\/pyqt-hello-world\/","title":{"rendered":"PyQt Hello World"},"content":{"rendered":"\n

Summary<\/strong>: in this tutorial, you’ll learn how to create a PyQt application that displays the Hello World message.<\/p>\n\n\n\n

Installing PyQt package #<\/a><\/h2>\n\n\n\n

PyQt is a Python binding for the Qt cross-platform application toolkit<\/a>. Because PyQt is a third-party package, you need to install it before use. <\/p>\n\n\n\n

PyQt6 is the latest version of PyQt at the time of writing this tutorial. To install PyQt6 using PyPI on Windows, you use the following pip<\/code> command:<\/p>\n\n\n

pip install PyQt6<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
If you use macOS or Linux, you need to use the pip3<\/code> command:<\/p>\n\n\n
pip3 install PyQt6<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
Creating the first PyQt program #<\/a><\/h2>\n\n\n\n
The following program displays a window with the title Hello World<\/code> on the screen:<\/p>\n\n\n
from<\/span> PyQt6.QtWidgets import<\/span> QApplication, QWidget\n\n# create the QApplication<\/span>\napp = QApplication([])\n\n# create the main window<\/span>\nwindow = QWidget(windowTitle='Hello World'<\/span>)\nwindow.show()\n\n# start the event loop<\/span>\napp.exec()<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
To run the application, you use the python<\/code> command:<\/p>\n\n\n
python main.py<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
If you use macOS or Linux, you can use the python3<\/code> command:<\/p>\n\n\n
python3 main.py<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
The following shows the window:<\/p>\n\n\n\n
\"\"<\/figure>\n\n\n\n
How it works.<\/p>\n\n\n\n
First, import<\/a> the QApplication<\/code> and QMainWidget<\/code> classes from the PyQt6.Widgets<\/code>:<\/p>\n\n\n
from<\/span> PyQt6.QtWidgets import<\/span> QApplication, QWidget<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
Second, create a QApplication<\/code> object:<\/p>\n\n\n
app = QApplication(sys.argv)<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
Each PyQt application needs one and only one<\/em> QApplication<\/code> object.<\/p>\n\n\n\n
Third, create a new instance of the QWidget<\/code> with the title 'Hello World'<\/code> and call the show()<\/code> method to display the window:<\/p>\n\n\n
window = QWidget(windowTitle='Hello World'<\/span>)\nwindow.show()<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
Finally, call the exec()<\/code> method of the QMainApplication<\/code> instance to start the event loop<\/em>:<\/p>\n\n\n
app.exec()<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
Introduction to the event loop #<\/a><\/h2>\n\n\n\n
Every PyQt application needs one instance of QApplication<\/code> class. The QApplication<\/code> object holds the event loop of the application. <\/p>\n\n\n\n
The event loop is responsible for managing all events of the application including user interactions with the GUI.<\/p>\n\n\n\n
When you interact with the Qt application e.g., by pressing a key or pushing a button, PyQt generates an event <\/em>and places it on an event queue<\/em>.<\/p>\n\n\n\n
The event loop continuously checks the event queue. If the event loop finds an event, it’ll forward the event to a specific event handler<\/em>. <\/p>\n\n\n\n
The event handler processes the event and passes the control back to the event loop for processing the next events. <\/p>\n\n\n\n
The following picture illustrates how the event loop works:<\/p>\n\n\n\n
\"PyQt<\/figure>\n\n\n\n
Note that each PyQt application has one and only one <\/em>event loop.<\/p>\n\n\n\n
Supporting command line argument & return code #<\/a><\/h2>\n\n\n\n
A PyQt program may accept one or more command line arguments. To enable this, you need to pass the argv<\/code> from the sys<\/code> module to the QApplication<\/code> like this:<\/p>\n\n\n
app = QApplication(sys.argv)<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
The app.exec()<\/code> returns 0<\/code> for success or other value for failure.<\/p>\n\n\n\n
If a PyQt program is executed from a Shell i.e., the command prompt on Windows or the Terminal on macOS and Linux, it should return a code indicating the program’s success or failure.<\/p>\n\n\n\n
To do this, you call the sys.exit()<\/code> function and pass the result of the app.exec()<\/code> to it as follows:<\/p>\n\n\n
sys.exit(app.exec())<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
 By doing this the Shell can receive the return code from the PyQt application.<\/p>\n\n\n\n
The following shows the revised Hello World<\/code> program:<\/p>\n\n\n
import<\/span> sys\nfrom<\/span> PyQt6.QtWidgets import<\/span> QApplication, QWidget\n\n\n# create the QApplication<\/span>\napp = QApplication(sys.argv)\n\n# create the main window<\/span>\nwindow = QWidget(windowTitle='Hello World'<\/span>)\nwindow.show()\n\n# start the event loop<\/span>\nsys.exit(app.exec())<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
Creating a Qt program template #<\/a><\/h2>\n\n\n\n
The following illustrates the PyQt Hello World in an object-oriented way:<\/p>\n\n\n\n
Note that if you are not familiar with object-oriented programming, you can follow the Python OOP tutorial<\/a>.<\/p>\n\n\n
import<\/span> sys\nfrom<\/span> PyQt6.QtWidgets import<\/span> QApplication, QWidget\n\n\nclass<\/span> MainWindow<\/span>(QWidget)<\/span>:<\/span>\n    def<\/span> __init__<\/span>(self, *args, **kwargs)<\/span>:<\/span>\n        super().__init__(*args, **kwargs)\n        \n        # set the window title<\/span>\n        self.setWindowTitle('Hello World'<\/span>)\n        \n        # show the window<\/span>\n        self.show()\n\n\nif<\/span> __name__ == '__main__'<\/span>:\n    app = QApplication(sys.argv)\n\n    # create the main window<\/span>\n    window = MainWindow()\n\n    # start the event loop<\/span>\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
First, define the MainWindow<\/code> class that inherits from the QWiget<\/code>:<\/p>\n\n\n
class<\/span> MainWindow<\/span>(QWidget)<\/span>:<\/span>\n    def<\/span> __init__<\/span>(self, *args, **kwargs)<\/span>:<\/span>\n        super().__init__(*args, **kwargs)\n        \n        # set the window title<\/span>\n        self.setWindowTitle('Hello World'<\/span>)\n        \n        # show the window<\/span>\n        self.show()<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
In the __init__()<\/code> method, we set the widow title and call the show()<\/code> method to display the window.<\/p>\n\n\n\n
if<\/span> __name__ == '__main__'<\/span>:<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
Then, create a QApplication<\/code> object:<\/p>\n\n\n
app = QApplication(sys.argv)<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
After that, create a new instance of the MainWindow<\/code>:<\/p>\n\n\n
window = MainWindow()<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
Finally, start the event loop:<\/p>\n\n\n
sys.exit(app.exec())<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
Summary #<\/a><\/h2>\n\n\n\n
  • Each PyQt application has one and only one QApplication<\/code> object. The QApplication<\/code> object holds an event loop.<\/li>
  • An event loop manages all events of the PyQt application. It checks the event queue continuously and forwards the events to their handlers.<\/li>
  • Call the app.exec()<\/code> to start the event loop.<\/li>
  • Use QMainWindow<\/code> to create the main window for the PyQt application and call the show()<\/code> method to display the window on the screen.<\/li><\/ul>\n
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Next, add the if __name__ == ‘__main__’<\/a> block to include the code if the program is executed directly. If the script is imported as a module, the code in this block will not be executed. <\/p>\n\n\n