Why are symbols not displaying after adding them via the Python Console?

Question

Steps to reproduce:

Create a new project->Add new Shapefile Layer(Polygon)->Click on the newly added Layer->Toggle Editing->Using the 'Add Feature' Tool draw a polygon and save the layer

Open the python console->Run the following lines of code in the python console

from PyQt4.QtGUI import QColor

layer = iface.activeLayer()
symbol = QgsLineSymbolV2.createSimple({})
symbol.deleteSymbolLayer(0)
symbol_layer = QgsSimpleLineSymbolLayerV2(QColor('red'), 2.0)
symbol.appendSymbolLayer(symbol_layer.clone())
layer.rendererV2().setSymbol(symbol.clone())
layer.triggerRepaint()
iface.legendInterface().refreshLayerSymbology(layer)

After running the above code the polygon stops displaying. Is this a bug in QGIS 2.18 or am I missing something?

Answer 1

I think you can't successfully apply a QgsSimpleLineSymbolLayerV2() symbol to a polygon because it belongs to linestring geometries.

The allowed symbols for polygons are:

• Simple fill (QgsSimpleFillSymbolLayerV2);
• Gradient fill (QgsFillSymbolLayerV2);
• Centroid fill (QgsCentroidFillSymbolLayerV2);
• Line pattern fill (QgsLinePatternFillSymbolLayer);
• Point pattern fill (QgsPointPatternFillSymbolLayer);
• SVG fill (QgsSVGFillSymbolLayer);
• Outline: marker line (QgsMarkerLineSymbolLayerV2);
• Outline: simple line (QgsSimpleLineSymbolLayerV2).

Answer 2

The problem with my code above is the symbol I was using. Once I changed the symbol to use the QgsLinePatternFillSymbolLayer object polygon displayed on the layer as expected.