Events and Signals in PySide

last modified October 18, 2023

In this part of the PySide programming tutorial, we explore events and signals occurring in applications.

Events

Events are an important part in any GUI program. Events are generated by users or by the system. When we call the application's exec_ method, the application enters the main loop. The main loop fetches events and sends them to the objects. PySide has a unique signal and slot mechanism.

All GUI applications are event-driven. An application reacts to different event types which are generated during its life. Events are generated mainly by the user of an application. But they can be generated by other means as well. e.g. Internet connection, window manager, timer. In the event model, there are three participants:

event source
event object
event target

The event source is the object whose state changes. It generates events. The event object (Event) encapsulates the state changes in the event source. The event target is the object that wants to be notified. Event source object delegates the task of handling an event to the event target.

When we call the application's exec_ method, the application enters the main loop. The main loop fetches events and sends them to the objects. Signals and slots are used for communication between objects. A signal is emitted when a particular event occurs. A slot can be any Python callable. A slot is called when a signal connected to it is emitted.

Signals & Slots

This is a simple example, demonstrating signals and slots in PySide.

#!/usr/bin/python

"""
ZetCode PySide tutorial 

In this example, we connect a signal
of a QtGui.QSlider to a slot 
of a QtGui.QLCDNumber. 

author: Jan Bodnar
website: zetcode.com
"""

import sys
from PySide import QtGui, QtCore

class Example(QtGui.QWidget):
    
    def __init__(self):
        super(Example, self).__init__()
        
        self.initUI()
        
    def initUI(self):
        
        lcd = QtGui.QLCDNumber(self)
        sld = QtGui.QSlider(QtCore.Qt.Horizontal, self)

        vbox = QtGui.QVBoxLayout()
        vbox.addWidget(lcd)
        vbox.addWidget(sld)

        self.setLayout(vbox)
        sld.valueChanged.connect(lcd.display)
        
        self.setGeometry(300, 300, 250, 150)
        self.setWindowTitle('Signal & slot')
        self.show()
        
def main():
    
    app = QtGui.QApplication(sys.argv)
    ex = Example()
    sys.exit(app.exec_())


if __name__ == '__main__':
    main()

In our example, we display a QtGui.QLCDNumber and a QtGui.QSlider. We change the lcd number by dragging the slider knob.

sld.valueChanged.connect(lcd.display)

Here we connect a valueChanged signal of the slider to the display slot of the lcd number.

The sender is an object that sends a signal. The receiver is the object, that receives the signal. The slot is the method that reacts to the signal.

Reimplementing event handler

Events in PySide are processed often by reimplementing event handlers.

#!/usr/bin/python

"""
ZetCode PySide tutorial 

In this example, we reimplement an 
event handler. 

author: Jan Bodnar
website: zetcode.com
"""

import sys
from PySide import QtGui, QtCore

class Example(QtGui.QWidget):
    
    def __init__(self):
        super(Example, self).__init__()
        
        self.initUI()
        
    def initUI(self):      
        
        self.setGeometry(300, 300, 250, 150)
        self.setWindowTitle('Event handler')
        self.show()
        
    def keyPressEvent(self, e):
        
        if e.key() == QtCore.Qt.Key_Escape:
            self.close()
        
def main():
    
    app = QtGui.QApplication(sys.argv)
    ex = Example()
    sys.exit(app.exec_())


if __name__ == '__main__':
    main()

In our example, we reimplement the keyPressEvent event handler.

def keyPressEvent(self, e):
    
    if e.key() == QtCore.Qt.Key_Escape:
        self.close()

If we click the escape button, the application terminates.

Event sender

Sometimes it is convenient to know, which widget is the sender of a signal. For this, PySide has a sender method.

#!/usr/bin/python

"""
ZetCode PySide tutorial 

In this example, we determine the event sender
object.

author: Jan Bodnar
website: zetcode.com
"""

import sys
from PySide import QtGui, QtCore

class Example(QtGui.QMainWindow):
    
    def __init__(self):
        super(Example, self).__init__()
        
        self.initUI()
        
    def initUI(self):      

        btn1 = QtGui.QPushButton("Button 1", self)
        btn1.move(30, 50)

        btn2 = QtGui.QPushButton("Button 2", self)
        btn2.move(150, 50)
      
        btn1.clicked.connect(self.buttonClicked)            
        btn2.clicked.connect(self.buttonClicked)
        
        self.statusBar()
        
        self.setGeometry(300, 300, 290, 150)
        self.setWindowTitle('Event sender')
        self.show()
        
    def buttonClicked(self):
      
        sender = self.sender()
        self.statusBar().showMessage(sender.text() + ' was pressed')
        
def main():
    
    app = QtGui.QApplication(sys.argv)
    ex = Example()
    sys.exit(app.exec_())


if __name__ == '__main__':
    main()

We have two buttons in our example. In the buttonClicked method we determine, which button we have clicked by calling the sender method.

btn1.clicked.connect(self.buttonClicked)            
btn2.clicked.connect(self.buttonClicked)

Both buttons are connected to the same slot.

def buttonClicked(self):
  
    sender = self.sender()
    self.statusBar().showMessage(sender.text() + ' was pressed')

We determine the signal source by calling the sender method. In the statusbar of the application, we show the label of the button being pressed.

Emitting signals

Objects created from QtCore.QObject can emit signals. If we click on the button, a clicked signal is generated. In the following example we see, how we can emit a custom signal.

#!/usr/bin/python

"""
ZetCode PySide tutorial 

In this example, we show how to emit a
signal. 

author: Jan Bodnar
website: zetcode.com
"""

import sys
from PySide import QtGui, QtCore

class Communicate(QtCore.QObject):
    
    closeApp = QtCore.Signal()

class Example(QtGui.QMainWindow):
    
    def __init__(self):
        super(Example, self).__init__()
        
        self.initUI()
        
    def initUI(self):      

        self.c = Communicate()
        self.c.closeApp.connect(self.close)       
        
        self.setGeometry(300, 300, 290, 150)
        self.setWindowTitle('Emit signal')
        self.show()
        
    def mousePressEvent(self, event):
        
        self.c.closeApp.emit()
        
def main():
    
    app = QtGui.QApplication(sys.argv)
    ex = Example()
    sys.exit(app.exec_())


if __name__ == '__main__':
    main()

We create a new signal called closeApp. This signal is emitted, during a mouse press event. The signal is connected to the close slot of the QtGui.QMainWindow.

class Communicate(QtCore.QObject):
    
    closeApp = QtCore.Signal()

We create a class based on a QtCore.QObject. It creates a closeApp signal when instantiated.

self.c = Communicate()
self.c.closeApp.connect(self.close)

An instance of the Communicate class is created. We connect a close slot of the QtGui.QMainWindow to the closeApp signal.

def mousePressEvent(self, event):
    
    self.c.closeApp.emit()

When we click on the window with a mouse pointer, the closeApp signal is emitted.