Signals and Slots

Introduction#

Functions or methods are executed in response to user’s actions like clicking on a button, selecting an item from a collection or a mouse click etc., called events.

Each PyQt widget, which is derived from QObject class, is designed to emit signal in response to one or more events. The signal on its own does not perform any action. Instead, it is connected to a slot.

Remarks#

In the following example, two QPushButton objects (b1 and b2) are added in QDialog window. We want to call functions b1_clicked() and b2_clicked() on clicking b1 and b2 respectively.

When b1 is clicked, the clicked() signal is connected to b1_clicked() function

b1.clicked.connect(b1_clicked())

When b2 is clicked, the clicked() signal is connected to b2_clicked() function

QObject.connect(b2, SIGNAL("clicked()"), b2_clicked)

Widgets used to build the GUI interface act as the source of such events.

Each PyQt widget, which is derived from QObject class, is designed to emit signal in response to one or more events. The signal on its own does not perform any action. Instead, it is connected to a slot. The slot can be any callable Python function.

An Example Using Signals and Slots

import sys
from PyQt4.QtCore import *
from PyQt4.QtGui import *

def window():
   app = QApplication(sys.argv)
   win = QDialog()
   b1 = QPushButton(win)
   b1.setText("Button1")
   b1.move(50,20)
   b1.clicked.connect(b1_clicked)

   b2 = QPushButton(win)
   b2.setText("Button2")
   b2.move(50,50)
   QObject.connect(b2,SIGNAL("clicked()"),b2_clicked)

   win.setGeometry(100,100,200,100)
   win.setWindowTitle("PyQt")
   win.show()
   sys.exit(app.exec_())

def b1_clicked():
   print ("Button 1 clicked")

def b2_clicked():
   print ("Button 2 clicked")

if __name__ == '__main__':
   window()

The following example will produce two buttons, if you click them, then function b1_clicked or b2_clicked will be called.