I got another issue writing a plugin to convert three shape files into an .obs input file. I created a UI (Qt) with a QComboBox to select a layer (that works fine). As soon as one of the layers in the QComboBox is selected, a second ComboBox should be filled with the fields of the layer to select from. To do that I'm trying to call a function, that is to be called, as soon, as a layer is selected in the first ComboBox. Unfortuntelly that does not work. Here is what I tried:

First I wrote a function to fill the second ComboBox with the field names:

  def build_Layer_Index_Changed():
    QMessageBox.information(None, "DEBUG", "Build Layer routine called. :)")
    canvas = self.iface.mapCanvas()
    dlg2 = ShowDialog2() 
    build = canvas.layer(int(dlg2.Sel_Build_Layer.currentIndex()))
    if int(dlg2.Sel_Build_Layer.currentIndex()) >= 0:
      if not build.geomTypeDict == 2:   # Types: 0:"point",1:"line",2:"polygon", Buildings layer has to be of type 2
        QMessageBox.critical(self.iface.mainWindow(), ":-/ Selected buillding layer is not of type vector(polygon)! :-/" ) 
        build_fields = self.getFields(build)

"dlg2.Sel_buildhight_field" is the second QComboBox. I tried to connect it to the first ComboBox's (dlg2.Sel_Build_Layer) Signal by

QtCore.QObject.connect(dlg2.Sel_Build_Layer, QtCore.SIGNAL("currentIndexChanged"), self.build_Layer_Index_Changed())

but I get an error: "TypeError: build_Layer_Index_Changed() takes no arguments (1 given)". If I remove the brackets after self.build_Layer_Index_Changed, I get no eror, but the function is not called (as the MessageBox doesn't appear. I thought I did it exactly as given in the Example at http://techbase.kde.org/Development/Tutorials/Python_introduction_to_signals_and_slots, but obviously I'm doing something wrong…

3 Answers 3


Try this:


Also the reason you are getting the error is because build_Layer_Index_Changed() doesn't take instance as it's first arg e.g. self. you need to have:

def build_Layer_Index_Changed(self):
  • I would also suggest watching/reading up on how Python works so you understand when/where you need to have self and brackets
    – Nathan W
    Oct 16, 2012 at 11:52

Try this while connecting: currentIndexChanged function pass 1 argument i.e is changed index and while defining build_Layer_Index_Changed pass two arguments (self, index).


After reading lots of tutorials and documantation, i changed my function to the following, which I think works (I was not able to run it yet due to another bug…).

def build_Layer_Index_Changed(self, canvas, dlg2, index):
    QMessageBox.information(None, "DEBUG", str("Build Layer routine called. :). Current Index = " + str(dlg2.Sel_Build_Layer.currentIndex()) + str(canvas) + str(dlg2) + str(build) + str(index)))
    if index >= 0:
        build = canvas.layer(index)
          build_fields = self.getFields(build)
          QMessageBox.critical(None, "Check selected layer!", ":-/ Faild to load buildings layer! (Index changed routine):-/" )

I call the function by

dlg2.Sel_Build_Layer.activated[int].connect(self.build_Layer_Index_Changed(canvas, dlg2))

which looks much more understandable to me than the C++like approach (Many thanks to Nathan!).

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