Qgis python to apply polygon Symbology

Question

I have polygon featureclass. which is shown in map as

I want to "outline xpattern" symbology

I am not able to do it programmatically by python.

Few of my attempts are: Attempt1:

layer = QgsProject.instance().mapLayersByName("New_Shapefile")[0] # replace "polygon_layer_name" with the actual name of your polygon layer
symbol = QgsFillSymbol.createSimple({'outline_style': 'no', 'outline_width': '0.26', 'outline_color': '0,0,0,255', 'pattern': '20'})
renderer = QgsSingleSymbolRenderer(symbol)
layer.setRenderer(renderer)
layer.triggerRepaint()

Attempt 2:

layer = QgsProject.instance().mapLayersByName("polygon_layer_name")[0] # replace "polygon_layer_name" with the actual name of your polygon layer
fill_symbol = QgsFillSymbol.createSimple({'color': '247,247,247,255'})
layer_1 = QgsFillSymbolLayer(fill_symbol)
layer_2 = QgsFillSymbolLayer(fill_symbol)
layer_3 = QgsFillSymbolLayer(fill_symbol)
symbol = QgsFillSymbol([layer_1, layer_2, layer_3])
renderer = QgsSingleSymbolRenderer(symbol)
layer.setRenderer(renderer)
layer.triggerRepaint()

Answer 1

You were close, you just needed QgsMarkerLineSymbolLayer for the outline points.

layer = QgsProject.instance().mapLayersByName("polygon_layer_name")[0] # replace "polygon_layer_name" with the actual name of your polygon layer
red = QColor(255,0,0)
green = QColor(0,255,0)
whiteish = QColor(247,247,247)
fill = QgsSimpleFillSymbolLayer()
fill.setColor(whiteish)
marker_r = QgsMarkerLineSymbolLayer(interval=9)
marker_r.setColor(red)
marker_r.setOffsetAlongLine(4.5)
marker_g = QgsMarkerLineSymbolLayer(interval=9)
marker_g.setColor(green)
fill_symbol = QgsFillSymbol([fill, marker_r, marker_g])
renderer = QgsSingleSymbolRenderer(fill_symbol)
layer.setRenderer(renderer)
layer.triggerRepaint()

UPDATE: you can also use the predefined style like this:

layer = QgsProject.instance().mapLayersByName("polygon_layer_name")[0] # replace "polygon_layer_name" with the actual name of your polygon layer
symbol = QgsStyle.defaultStyle().symbol("outline xpattern")
layer.renderer().setSymbol(symbol)
layer.triggerRepaint()