PyQt5 – QApplication

25 Aug, 2021
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 sys  class 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("GeeksforGeeks", 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 
sumitgumber28
Python-PyQt
Python
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