[GIS] Modifying style (line width/color) of vector layer in PyQGIS

Question

I'm currently updating some python code from QGIS 2.18 to QGIS 3.4

in 2.18 i used the following code :

#First off I created a virtual layer using an sql query here

vlayer = QgsVectorLayer( "?query=...., "virtual" )

# Then I assign the properties of the polyline layer here

props = { 'width' : '3', 'color' : '255,0,0' } 
sl = QgsSymbolLayerV2Registry.instance().symbolLayerMetadata("SimpleLine").createSymbolLayer(props)
s = QgsLineSymbolV2([sl]) vlayer.setRendererV2( QgsSingleSymbolRendererV2( s ) )

I know there have been a number of changes in QGIS 3 for exempleQgsSymbolLineSymbolV has become QgsLineSymboland setRendererV2 which becomes setRenderer. So, with the help of this blog this GitHubthe API and the most recent doc on the QGIS site, I've put the script below together…

layer = QgsProject.instance().mapLayersByName("t_alpha")[0]

single_symbol_renderer = layer.renderer()

symbol = single_symbol_renderer.symbol()

symbol.setColor(QColor.fromRgb(255, 0, 0))
symbol.setWidth(3)


layer.triggerRepaint()

qgis.utils.iface.layerTreeView().refreshLayerSymbology(layer.id())

But this returns an error:

AttributeError: 'QgsCategorizedSymbolRenderer' object has no attribute
'symbol'

I've dug through the doc and otherwise tried to remodulate the code a bit but I'm a bit stuck (i'm still learning Python).

Answer 1

In QGIS 3.4.x is:

layer = QgsProject.instance().mapLayersByName("t_alpha")[0]
layer.renderer().symbol().setWidth(0.7)
layer.triggerRepaint()