121 lines
3.7 KiB
Markdown
121 lines
3.7 KiB
Markdown
---
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author: einar
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comments: true
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date: 2010-03-06 08:04:48+00:00
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layout: post
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slug: pykde4-new-style-signals-and-slots
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title: 'PyKDE4: new style signals and slots'
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wordpress_id: 763
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categories:
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- KDE
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- Linux
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tags:
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- KDE
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- programming
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---
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Those who use PyQt and PyKDE4 are certainly familiar with the syntax used to connect signals and slots:
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[python]
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from PyQt4 import QtCore
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from PyQt4 import QtGui
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from PyKDE4 import kdeui
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class MyGUI(QtGui.QWidget):
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def __init__(self, parent=None):
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super(MyGUI, self).__init__(parent)
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self.pushbutton = kdeui.KPushButton()
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self.pushbutton.setText("Push me!")
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QObject.connect(self.pushbutton, QtCore.SIGNAL("clicked()"),
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self.button_pushed)
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def button_pushed(self):
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print "Button clicked"
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[/python]
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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:
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* 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;
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* In case you have overloaded signals, you have to type the exact signature, going back to the first problem;
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* It's not Pythonic at all.
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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:
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[python]
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from PyQt4 import QtCore
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from PyQt4 import QtGui
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from PyKDE4 import kdeui
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class MyGUI(QtGui.QWidget):
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def __init__(self, parent=None):
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super(MyGUI, self).__init__(parent)
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self.pushbutton = kdeui.KPushButton()
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self.pushbutton.setText("Push me!")
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# New style
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self.pushbutton.clicked.connect(self.button_pushed)
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def button_pushed(self):
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print "Button clicked"
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[/python]
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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.
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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):
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[python]
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# One signal is without arguments, the other has a bool
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# Signal without arguments
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self.my_widget.connected.connect(self.handle_errors)
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# Signal with a book
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self.my_widget.connected[bool].connect(self.handle_errors)
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[/python]
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Signals are emitted with the emit() function and disconnected with the disconnect() function:
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[python]
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# Emit a signal
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self.pushbutton.clicked.emit()
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# Emit a signal with a value (an int)
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self.my_widget.valueChanged.emit(int)
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# Disconnect another
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self.my_tabwidget.currentIndexChanged.disconnect()
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[/python]
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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:
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[python]
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class MyWidget(QWidget):
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# Signal with no arguments
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operationPerformed = QtCore.pyqtSignal()
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# Signal that takes arguments
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valueChanged = QtCore.pyqtSignal(int)
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[/python]
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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.
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