---
author: einar
comments: true
date: 2010-03-06 08:04:48+00:00
layout: post
slug: pykde4-new-style-signals-and-slots
title: 'PyKDE4: new style signals and slots'
wordpress_id: 763
categories:
- KDE
- Linux
tags:
- KDE
- programming
---

Those who use PyQt and PyKDE4 are certainly familiar with the syntax used to connect signals and slots:



[python]
from PyQt4 import QtCore
from PyQt4 import QtGui
from PyKDE4 import kdeui

class MyGUI(QtGui.QWidget):

    def __init__(self, parent=None):
        super(MyGUI, self).__init__(parent)
        self.pushbutton = kdeui.KPushButton()
        self.pushbutton.setText("Push me!")
        
        QObject.connect(self.pushbutton, QtCore.SIGNAL("clicked()"), 
                               self.button_pushed)

    def button_pushed(self):
        print "Button clicked"
[/python]


The main advantage of this syntax is that it's very close to the C++ equivalent, and so you can translate easily from C++ to Python. Unfortunately the advantages of this syntax end here. The disadvantages, at least from a Python coding perspective, outweigh the advantages:

  * It's _extremely_ error-prone: make a typo, and not only your signal won't be connected, but you won't even get a warning, your program will simply do nothing;
  * In case you have overloaded signals, you have to type the exact signature, going back to the first problem;
  * It's not Pythonic at all.

So, in recent PyQt versions (and thus also in PyKDE4) a _new style_ approach was introduced (although the old style is always present should it be the need to). Using the new style, the signals become a property of the object that emits them. and then you use the connect function of that property. Here's the example using the new style-signals:



[python]
from PyQt4 import QtCore
from PyQt4 import QtGui
from PyKDE4 import kdeui

class MyGUI(QtGui.QWidget):

    def __init__(self, parent=None):
        super(MyGUI, self).__init__(parent)
        self.pushbutton = kdeui.KPushButton()
        self.pushbutton.setText("Push me!")
        # New style
        self.pushbutton.clicked.connect(self.button_pushed)

    def button_pushed(self):
        print "Button clicked"
[/python]




As you can see it's much clearer, and much more Pythonic. Also, typos  **will** trigger an AttributeError, which means you'll be able to track where the problem is.  





What about overloaded signals? Normally the first defined is the default, but you can use a dictionary-like syntax to access other overloads (signal names are completely made up here):



[python]
# One signal is without arguments, the other has a bool

# Signal without arguments
self.my_widget.connected.connect(self.handle_errors)
# Signal with a book
self.my_widget.connected[bool].connect(self.handle_errors)
[/python]

Signals are emitted with the emit() function and disconnected with the disconnect() function:

[python]
# Emit a signal
self.pushbutton.clicked.emit()
# Emit a signal with a value (an int)
self.my_widget.valueChanged.emit(int)
# Disconnect another
self.my_tabwidget.currentIndexChanged.disconnect()
[/python]



To define new signals, you can use the _pyqtSignal_ function, specifying which values will the signal take (if any): just define that as a class constant (like in the example) and then you can access them like the wrapped ones:



[python]

class MyWidget(QWidget):
    
    # Signal with no arguments
    operationPerformed = QtCore.pyqtSignal()

    # Signal that takes arguments
    valueChanged = QtCore.pyqtSignal(int)
[/python]



I merely scratched the surface with this. For more information, check out [PyQt's reference manual](http://riverbankcomputing.co.uk/static/Docs/PyQt4/pyqt4ref.html), which also covers other cases.
