Removing legend items with PyQGIS without removing the layer

Question

I am trying to generate maps in PDF files by using a pre-defined .qgz layout. I am already able to load layers and adjust color scale, etc. Since the legend is synchronized with the map layers, all the layers I have loaded are listed in the legend.

Using PrintComposer in QGIS I can turn off synchronization and remove the unwanted items in legend. Is there a way to do it automatically in Python? I tried to use QgsLegendModel, but it also removes the layers from map canvas.

.

projectInstance= QgsProject.instance()
projectLayoutManager = projectInstance.layoutManager()

for comp in projectLayoutManager.printLayouts():
    for item in comp.items():
        if(isinstance(item, QgsLayoutItemLegend)):                
            model = item.model()
            while model.rowCount() > 1:
                 model.removeRow(0)
                      
            break

Answer 1

This is code adapted for QGIS 3 from this answer. It adds all layers except one specified. You could swap this out for a list, or conversely, use the layers you want to keep in rather than those you want to leave out.

l = QgsProject.instance().layoutManager()
    
for layout in l.printLayouts():    
    legend = QgsLayoutItemLegend(layout)
    root = QgsLayerTree()
    for lyr in iface.mapCanvas().layers():
        if lyr.name() != 'Unwanted layer':  
            root.addLayer(lyr)
        legend.model().setRootGroup(root)
        layout.addItem(legend)

In place of if lyr.name() != 'Unwanted layer': you could use

if lyr.name() not in [list, of, unwanted, layer, names]:

or

if lyr.name() in [list, of, wanted, layer, names]:

Answer 2

@Matt, your solution started to work after I've forced qt to process the buffered events, before iterating through layers. Thank you for your hint.

from PyQt5 import QtWidgets

inst = QtWidgets.QApplication.instance()
qapp = QtWidgets.qApp
qapp.processEvents()

l = QgsProject.instance().layoutManager()

for layout in l.printLayouts():    
    legend = QgsLayoutItemLegend(layout)
    root = QgsLayerTree()
    for lyr in iface.mapCanvas().layers():
        if lyr.name() != 'Unwanted layer':  
            root.addLayer(lyr)
        legend.model().setRootGroup(root)
        layout.addItem(legend)

Answer 3

May be not a direct answer to a question but I think my case could also be met by someone else.

I was trying to find a way to remove items of layers in composer legend. If layer has a rule-based or unique values symbology type sometimes you need to filter them in a legend. I didn’t want to reclassify or re-order symbology of initial layer, just keep it permanent but change it in composer’s legend. In general, I wanted to emulate a minus button click in legend settings.

Recently I have found a solution thanks to another answer from StackOverflow: https://stackoverflow.com/a/67223630/8776330.

It is based on use of QgsMapLayerLegendUtils and legend model. In my case there is a single layout with only one legend item containing two layers. The first layer multi_layer2 has four symbology nodes and the goal is to remain only two of them.

Code:

# define layer which legend has to be modified
layer = QgsProject.instance().mapLayersByName('multi_layer2')[0]

# get a layout and legend item
project = QgsProject.instance()         
layout_manager = project.layoutManager()
layouts_list = layout_manager.printLayouts()
layout = layouts_list[0]
legend = next(filter(lambda n: type(n)==QgsLayoutItemLegend, layout.items()))

# get legend items 
model = legend.model()
layer_tree = legend.model().rootGroup().findLayer(layer) # QgsLayerTreeLayer object
symbols = model.layerOriginalLegendNodes(layer_tree) # collection of layer legend nodes

# assuming we want to keep only first and third node
symbols_to_remain = [0,2]

# setting a new order and applying changes
QgsMapLayerLegendUtils.setLegendNodeOrder(layer_tree, symbols_to_remain)
model.refreshLayerLegend(layer_tree)

Result: