Symbology

Simple symbology on points

my_layer = QgsProject.instance().mapLayersByName("My points")[0]
 
symbol_peak = QgsMarkerSymbol.createSimple({'name': 'Triangle', 'color': 'green', 'outline_color': 'black', 'size': '4'})
mylayer.renderer().setSymbol(symbol_peak)
mylayer.triggerRepaint()

Other point markers: 

circle square cross rectangle diamond pentagon triangle equilateral_triangle star regular_star arrow filled_arrowhead x

Simple symbology on lines

With HTML color.

myroads = QgsProject.instance().mapLayersByName("Roads")[0]
myrivers = QgsProject.instance().mapLayersByName("Rivers")[0]
 
# Roads
symbol = QgsLineSymbol.createSimple({'line_style': 'dash', 'line_width': '0.5', 'color': 'black'})
myroads.renderer().setSymbol(symbol)
myroads.triggerRepaint()
 
# Rivers
symbol = QgsLineSymbol.createSimple({'line_style': 'solid', 'line_width': '0.75', 'color': '#0088CC'})
myrivers.renderer().setSymbol(symbol)
myrivers.triggerRepaint()

Simple symbology on polygons

With RGB color (50,165,77,75), and some transparency (here 75, from 0 to 255), et on rafraîchit le panneau des couches afin que notre légende y soit aussi prise en compte:

myground = QgsProject.instance().mapLayersByName("Ground")[0]
 
symbol = QgsFillSymbol.createSimple({'line_style': 'solid', 'line_width': '0.2', 'color': '50,165,77,75'})
myground.renderer().setSymbol(symbol)
 
myground.triggerRepaint()
iface.layerTreeView().refreshLayerSymbology(myground.id())
iface.mapCanvas().refresh()

Check current symbology properties

my_layer = QgsProject.instance().mapLayersByName("My points")[0]
print(my_layer.renderer().symbol().symbolLayers()[0].properties())

Categorized Symbology

For a categorized symbology, we will need to define several symbologies, then assign them based on attribute values.

Here we put all our peaks in blue triangle (▲, then we will look for a single peak based on its name (My best peak, in the NAME field). This will change to a dark blue triangle and larger size (▲).

symbol_peak = QgsMarkerSymbol.createSimple({'name': 'Triangle', 'color': '#0088CC', 'outline_color': 'black', 'size': '4'})
symbol_peak_selected = QgsMarkerSymbol.createSimple({'name': 'Triangle', 'color': 'blue', 'outline_color': 'black', 'size': '7'})
 
color_peak = QgsRendererCategory(None, symbol_peak, 'Peaks', True)
color_peak_selected = QgsRendererCategory('My best peak', symbol_peak_selected, 'My best peak', True)
 
renderer = QgsCategorizedSymbolRenderer("NAME", [color_peak,color_peak_selected])
 
mylayer.setRenderer(renderer)
mylayer.triggerRepaint()
iface.mapCanvas().refresh()

Transparency on a basemap (OSM)

osm = QgsRasterLayer(urlWithParams, "OpenStreetMap", "wms")
project.addMapLayer(osm)
osm.setOpacity(0.75)
 
iface.mapCanvas().refresh()