Append Symbol Layer PyQGIS3

Question

I am updating a QGIS2 plugin to QGIS3. The following code in PyQGIS2 works:

        association_symbol = QgsSymbolLayerV2Registry.instance().symbolLayerMetadata("ArrowLine").createSymbolLayer({
        'arrow_width': '0.4',
        'head_length': '2.0',
        'head_thickness' : '0.75',
        'color': 'red',
        'line_color': 'red',
        'is_curved': '0',
        'arrow_start_width': '.2'
    })
    self.road_object_path_association_layer.rendererV2().symbols()[0].appendSymbolLayer(association_symbol)

I made an attempt at correcting for PyQGIS3, but it does not work:

        association_symbol = QgsApplication.symbolLayerRegistry().symbolLayerMetadata("ArrowLine").createSymbolLayer({
        'arrow_width': '0.4',
        'head_length': '2.0',
        'head_thickness' : '0.75',
        'color': 'red',
        'line_color': 'red',
        'is_curved': '0',
        'arrow_start_width': '.2'
    })
    self.road_object_path_association_layer.renderer().symbol().symbolLayers()[0].appendSymbolLayer(association_symbol)

The error returned is as follows:

AttributeError: 'QgsSimpleLineSymbolLayer' object has no attribute 'appendSymbolLayer'

The symbols() object required an argument. I was not sure what argument to provide. Further research let me to believe I could get the info from the symbolLayers()[0] object, but that appears to have been a red herring. Any assistance would be helpful.

Answer 1

You don't need symbolLayers()[0] this,

road_object_path_association_layer.renderer().symbol()

is a QgsLineSymbol that inherits from QgsSymbol and here you find the method appendSymbolLayer

Code:

association_symbol = QgsApplication.symbolLayerRegistry().symbolLayerMetadata("ArrowLine").createSymbolLayer({
        'arrow_width': '0.4',
        'head_length': '2.0',
        'head_thickness' : '0.75',
        'color': 'red',
        'line_color': 'red',
        'is_curved': '0',
        'arrow_start_width': '.2'
    })
road_object_path_association_layer.renderer().symbol().appendSymbolLayer(association_symbol)
road_object_path_association_layer.triggerRepaint() # Add repaint layer

Class Reference