Set offset for rule based styling via PyQGIS

Question

I'm trying to programatically render bunch of vector lines that have an attribute 'route_id' which specifies which route they belong to, as well as an attribute 'ranking'. Some features overlap, and the 'ranking' attribute specifies the order in which they should be offset (ranking==1 -> offset by n * 1, ranking==2 -> offset by n * 2 etc.) so via rule-based symbology it should be possible to render them the way I need.

I'm however struggling to access the offset parameter via PyQGIS. As far as I can tell there are only methods for setting width and color.

layer = iface.activeLayer()
symbol = QgsSymbol.defaultSymbol(layer.geometryType())
renderer = QgsRuleBasedRenderer(symbol)
expression_1 = '"route_id" = \'109\' AND "ranking" = \'2\''
width = 0.86

def rule_based_style(layer, symbol, renderer, label, expression, color, width):
    root_rule = renderer.rootRule()
    rule = root_rule.children()[0].clone()
    rule.setLabel(label)
    rule.setFilterExpression(expression)
    rule.symbol().setColor(QColor(color))
    rule.symbol().setWidth(width)
    root_rule.appendChild(rule)
    layer.setRenderer(renderer)
    layer.triggerRepaint()
    iface.layerTreeView().refreshLayerSymbology(layer.id())
    
rule_based_style(layer, symbol, renderer, 'Rule_1', expression_1, 'red', width)

Is there any way I can specify the offset?

Answer 1

The symbol().setColor() and symbol.setWidth() are high level convenience methods. To access many more styling options you need to dig a bit deeper.

To elaborate on @JULESG's comment, you can access the individual symbol layers of the rule.symbol().

lyr = iface.activeLayer()

renderer = lyr.renderer()

root_rule = renderer.rootRule()

# here I don't clone the rule because I want to make changes in-place.
rule = root_rule.children()[0]

sym = rule.symbol()

# get the (first) symbol layer of the symbol
s = sym.symbolLayers()[0]

# set the offset. Here it is arbitrary, but you can set this dynamically in your code
s.setOffset(5)

# update the changes on the map canvas
lyr.triggerRepaint()

Note: The black line here is just for visualisation (a duplicate of the original layer). You will only see the offset line using the code above.