How is the function launched without a function call?

Question

Can somebody explain to me how the program knows that if I click the right mouse button it must launch the contextMenuEvent? Where is this call made? And if it is the parent class that is doing it, how it knows the name of the function?

import sys
from PyQt5.QtWidgets import QMainWindow, qApp, QMenu, QApplication

class Example(QMainWindow):

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

        self.initUI()


    def initUI(self):         

        self.setGeometry(300, 300, 300, 200)
        self.setWindowTitle('Context menu')    
        self.show()


    def contextMenuEvent(self, event):

           cmenu = QMenu(self)

           newAct = cmenu.addAction("New")
           opnAct = cmenu.addAction("Open")
           quitAct = cmenu.addAction("Quit")
           action = cmenu.exec_(self.mapToGlobal(event.pos()))

           if action == quitAct:
               qApp.quit()

if __name__ == '__main__': 
    app = QApplication(sys.argv)
    ex = Example()
    sys.exit(app.exec_())

Answer 1

QMainWindow already provides the contextMenuEvent method which is called by the Qt framework.

In your class, you are overriding this method so that your method is called instead of the one from QMainWindow.

See also

• QMainWindow::contextMenuEvent()
• Qt Documentation: Menus Example