Create rule-based renderer from an existing renderer using PyQGIS

Question

I want to make a copy from an existing layer in the legend and render it with a rule-based renderer based on the original layer's graduated symbol renderer. Then I will change its rule expressions, clone all the rules (if that is even possible) and change again their expressions.

Right now I'm at step one: creating a rule-based renderer from an existing graduated symbol renderer. With iface.actionDuplicateLayer().trigger() I clone the original layer with its renderer. Then I use convertFromRenderer function to create a QgsRuleBasedRendererV2 from the new layer's graduated symbol renderer and I assign it to the layer. But there's something I'm doing wrong because QGIS crashes at this point and I can't find the reason.

# Duplicate original layer
for layer in iface.legendInterface().layers():
 if layer.name() == 'Lines':
 iface.setActiveLayer(layer)
 iface.actionDuplicateLayer().trigger()
# Make the new layer visible and change its name
for layer in iface.legendInterface().layers():
 if layer.name() == 'Lines copy':
 vl = layer
iface.legendInterface().setLayerVisible(vl, True)
vl.setLayerName('Lines - Atlas')
# Create rule-based renderer from its graduated symbol renderer
# and assign it to it
renderer = vl.rendererV2()
if renderer.type() == 'graduatedSymbol':
 rSimbol = renderer.sourceSymbol()
 ruleRend = QgsRuleBasedRendererV2(rSimbol).convertFromRenderer(renderer)
 vl.setRendererV2(ruleRend) #<------- QGIS crashes here
 iface.legendInterface().refreshLayerSymbology(vl)
#Refresh canvas
canvas = iface.mapCanvas()
canvas.refresh()

• Matthias Kuhn: Does that work better? QgsRuleBasedRendererV2.convertFromRenderer(renderer)
• DMS02: Thank you so much @MatthiasKuhn. That has sense. There is no need to provide a default symbol to the renderer since it will take everything from the original renderer. I don't know why I discarded that possibility.
• Matthias Kuhn: Great, turned it into an answer ;)

Answer 1

convertFromRenderer is a static function and therefore does not need an instance to work on.

You can simply use the following:

if renderer.type() == 'graduatedSymbol':
 ruleRend = QgsRuleBasedRendererV2.convertFromRenderer(renderer)
 ...