The QApplication class manages the GUI application’s control flow and main settings. It specializes in the QGuiApplication with some functionality needed for QWidget based applications. It handles widget specific initialization, finalization. For any GUI application using Qt, there is precisely one QApplication object, no matter whether the application has 0, 1, 2, or more windows at any given time. For non-QWidget based Qt applications, use QGuiApplication instead, as it does not depend on the QtWidgets library.
We then create a window instance and execute the QApplication object in the event loop using sys.exit(App.exec()) command, below are some useful and frequently methods and property used with the QApplication object.
Syntax: App = QApplication(sys.argv)
Parameters:
- beep: Sounds the bell, using the default volume and sound. This function is not available in Qt for Embedded Linux
- setFont: It sets the default font of the PyQt5 Application
- aboutQt: Displays a simple message box about Qt. The message includes the version number of Qt being used by the application.
- closeAllWindows: Closes all top-level windows. This function is particularly useful for applications with many top-level windows.
- setAutoSipEnabled: It automatically displays the SIP when entering widgets that accept keyboard input
- setCursorFlashTime: This method sets the text cursor’s flash (blink) time in milliseconds
- setDoubleClickInterval: This method sets the time limit in milliseconds that distinguishes a double click from two consecutive mouse clicks
Example:
We will create a simple PyQt5 application which produces a beep sound when it gets executed and many properties are set to the QApplication object, below is the implementation
Python3
# importing librariesfrom PyQt5.QtWidgets import *from PyQt5 import QtCore, QtGuifrom PyQt5.QtGui import *from PyQt5.QtCore import *import sysclass Window(QMainWindow): def __init__(self): super().__init__() # setting title self.setWindowTitle("Python ") # setting geometry self.setGeometry(100, 100, 500, 400) # calling method self.UiComponents() # showing all the widgets self.show() # method for components def UiComponents(self): # creating a push button push = QPushButton("Press", self) # setting geometry to the push button push.setGeometry(100, 100, 120, 40) # creating a label label = QLabel("Lazyroar", self) # setting geometry to the label label.setGeometry(100, 160, 200, 50) # setting alignment to the label label.setAlignment(Qt.AlignCenter) # font font = QFont("Arial", 12) # setting font to the label label.setFont(font) # setting style sheet to the label label.setStyleSheet("QLabel" "{" "border : 2px solid green;" "background : lightgreen;" "}") # hiding the label label.hide() # adding action method to the push button push.clicked.connect(lambda: do_something()) # method called by the push button when pressed def do_something(): # unhide the label label.show()# create pyqt5 appApp = QApplication(sys.argv)# setting cursor flashtimeApp.setCursorFlashTime(100)# setting application object nameApp.setObjectName("GfG")# setting application display nameApp.setApplicationDisplayName("GfG PyQt5")# beep sound will occur when application# is openedApp.beep()# message displayed about the QtApp.aboutQt()# create the instance of our Windowwindow = Window()# start the appsys.exit(App.exec()) |
Output:
When we execute the code firstly the about Qt page will get displayed
Then our application will get started
