I'm three years late, but hopefully whoever else might be stuck on an issue like this will benefit from my answer.

 # adds a LineLayerStyle (Arrow, dotted)  
 # layer being a vector layer object of QgsVectorLayer()
   def setLineLayerStyle(self, layer, style):  
     if(style == "arrow"):  
         sl = QgsSymbolLayerV2Registry.instance().symbolLayerMetadata("LineDecoration").createSymbolLayer({ 'width' : '0.26', 'color' : '0,0,0' })  
         symbollist = layer.rendererV2().symbols()  
         symbol = symbollist[0]  
         symbol.appendSymbolLayer(sl) 

Following this code, you will get a line that looks like the pink arrow. All of this information is from here

 # adds a LineLayerStyle (Arrow, dotted)  
 # layer being a vector layer object of QgsVectorLayer()
   def setLineLayerStyle(self, layer, style):  
     if(style == "arrow"):  
         sl = QgsSymbolLayerV2Registry.instance().symbolLayerMetadata("LineDecoration").createSymbolLayer({ 'width' : '0.26', 'color' : '0,0,0' })  
         symbollist = layer.rendererV2().symbols()  
         symbol = symbollist[0]  
         symbol.appendSymbolLayer(sl) 

Following this code, you will get a line that looks like the pink arrow. All of this information is from here