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I am updating a QGIS Plugin from QGIS 2 to 3 and I am struggling with changing the layer symbology (a single symbol type for a line layer). I would like to have all lines in my layer to have the same color and the same thickness. For QGIS 2 it worked like this:

symbols = iface.activeLayer().rendererV2().symbols()
symbol = symbols[0]
symbol.setColor(QColor.fromRgb(0, 225, 0))
symbol.setWidth(0.75)
qgis.utils.iface.mapCanvas().refresh()
qgis.utils.iface.legendInterface().refreshLayerSymbology(iface.activeLayer())

For QGIS 3, I understood from the API break documentation that rendererV2() has been renamed to renderer(). Further, symbols() has been removed:

symbols( QgsRenderContext& context ) method should be used instead (previously available as symbols2 in PyQGIS bindings).

I find it hard to connect this information and what I find under the QgsRenderContext documentation to my problem.

An explanation on how to solve such issue based on the documentations would be helpful.

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  • The correct answer depends on your renderer type - is this single symbol, categorized, or something else?
    – ndawson
    Commented May 25, 2018 at 0:53
  • @ndawson: a single symbol type for a line layer. I would like to have all lines in my layer to have the same color and the same thickness. I will add this info to the question.
    – Sophie Crommelinck
    Commented May 25, 2018 at 5:39

1 Answer 1

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If it's a single symbol renderer, then something like:

layer = iface.activeLayer()
single_symbol_renderer = layer.renderer()

symbol = single_symbol_renderer.symbol()
symbol.setColor(QColor.fromRgb(0, 225, 0))
symbol.setWidth(0.75)

# more efficient than refreshing the whole canvas, which requires a redraw of ALL layers
layer.triggerRepaint()

# update legend for layer
qgis.utils.iface.layerTreeView().refreshLayerSymbology(layer.id())
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  • Further, a short explanation on how to solve such issue based on the documentations would be helpful. E.g. starting with the QgsVectorLayer, I see that from the classQgsVectorLayer I can retrieve with renderer() an object of the classQgsFeatureRenderer. However, there I see no function symbol() to retrieve a classQgsMarkerSymbol, which I need to then use setColor() etc.
    – Sophie Crommelinck
    Commented May 25, 2018 at 6:52
  • 1
    You need to also check the subclasses - in this case calling renderer() on the layer will give you the subclass, a QgsSingleSymbolRenderer. Checking qgis.org/api/classQgsSingleSymbolRenderer.html shows the symbol/setSymbol methods.
    – ndawson
    Commented May 25, 2018 at 7:26

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