PyKDE4: new style signals and slots

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, which also covers other cases.

Dialogue & Discussion