PyQGIS – Legend Doesn’t Update with Renderer

Question

I am using QGIS 3.22.2.

I am relatively new to QGIS and have been trying to automate part of a workflow, including changing the rendering of a raster. Following the PyQGIS cookbook, I have this code:

stats = rlayer.dataProvider().bandStatistics(1, QgsRasterBandStats.All)
min = stats.minimumValue
max = stats.maximumValue

fnc = QgsColorRampShader()
fnc.setColorRampType(QgsColorRampShader.Interpolated)

lst = [  QgsColorRampShader.ColorRampItem(min, QColor(255, 0, 0)),
  QgsColorRampShader.ColorRampItem(max, QColor(0, 0, 255))]
  
fnc.setColorRampItemList(lst)

shader = QgsRasterShader()
shader.setRasterShaderFunction(fnc)

renderer = QgsSingleBandPseudoColorRenderer(rlayer.dataProvider(), 1, shader)

rlayer.setRenderer(renderer)
rlayer.triggerRepaint()

This works grand, and changes how a selected raster renders as shown below.

However, the legend doesn't update as expected. In this case, instead of an interpolated color bar between red and blue and specified values min–max, I get an almost completely blue color bar (the last color in my variable lst) between values 0-255. This is shown in the image below. The left hand legend is what I get when manually changing the rendering, and the right hand image is the result when running the above code.

How might I go about updating the legend programmatically?

Answer 1

You can use the script below (just change your layer name in the first line):

layer_name = 'your_layer_name'
rl = QgsProject.instance().mapLayersByName(layer_name)[0]
prov = rl.dataProvider()
stats = prov.bandStatistics(1, QgsRasterBandStats.All, rl.extent(), 0)
min = stats.minimumValue
max = stats.maximumValue
renderer = QgsSingleBandPseudoColorRenderer(rl.dataProvider(), 1)
color_ramp = QgsGradientColorRamp(QColor(255, 0, 0), QColor(0, 0, 255))
renderer.setClassificationMin(min)
renderer.setClassificationMax(max)
renderer.createShader(color_ramp)
rl.setRenderer(renderer)
rl.triggerRepaint()

Results on a test DEM layer:

You can also pass different ramp types and classifaction modes, along with a number of classes to the createShader() method. For example, here I reversed the color ramp and changed this line:

renderer.createShader(color_ramp)

To:

renderer.createShader(color_ramp, QgsColorRampShader.Discrete, QgsColorRampShader.EqualInterval, 15)

Results:

Docs for QgsSingleBandPseudoColorRenderer

I also got some hints by looking through the Python unit tests for QgsRasterLayer here.