Modifying layer name in the layout legend with PyQGIS 3

Question

I would like to update the legend item in my layout with other names.

I tried the following code, which returns no error but layers name remain the same.

project = QgsProject.instance()
layout = QgsPrintLayout(project)
layout.initializeDefaults()

layersToAdd = [puit, puit_m, amas, home, fosse, eau_l, eau_p, pdf] # each layer is defined with mapLayersByName
Names = ['Puit', 'Puit municipal', 'Amas', 'Maison', 'Fossé', 'Cours d\'eau', 'Surface d\'eau', 'Champs']
legend = QgsLayoutItemLegend(layout)
legend.setAutoUpdateModel(False)
root = QgsLayerTree()
tree=legend.model().rootGroup()
i = 0
for layer in layersToAdd:
    #add layer objects to the layer tree
    root.addLayer(layer)
    tree.children()[i].setName(Names[i])
    i += 1
legend.updateLegend()
legend.model().setRootGroup(root)
layout.addLayoutItem(legend)

Answer 1

You must set setUseLayerName() to False to name legend tree layers manually.

#
# previous lines
#

for i, layer in enumerate(layersToAdd):
    tree_layer = root.addLayer(layer)
    tree_layer.setUseLayerName(False)
    tree_layer.setName(names[i])
    
legend.updateLegend()

...

Answer 2

There's another way, figured out with heavy inspiration from the QGIS API on QgisLayoutItemLegend:

legend = QgsLayoutItemLegend(layout)
legend.setAutoUpdateModel(False)  # legend derived from the map's layers but now unlinked

for tr in legend.model().rootGroup().children():
    if tr.name() == layer_name:
        tr.setCustomProperty("legend/title-label", "new name in legend")

layout.addLayoutItem(legend)

Answer 3

In addition to Kadir Şahbaz answer.

According to comment section, seb007 noticed that names may not be eventually changed if there is no QgsLayerTree update for a legend object. As he posted, there should be a re-setting of legend root:

root=legend.model().rootGroup()
legend.model().setRootGroup(root)

Also in my case I have multiple legend objects in multi-page layout. My experience, github comments and some snippets shown that each QgsLayerTree value for each QgsLayoutItemLegend should be named individually. I.e. if you have a list of legends, QgsLayerTree values have to be different like

self.tree1 = QgsLayerTree()
self.tree2 = QgsLayerTree()
...
self.legend_obj1.model().setRootGroup(self.tree1)
self.legend_obj2.model().setRootGroup(self.tree2)

If the number of legends is different or you want this process to be in a loop, I may recommend to use settattr() and getattr() functions in a class.

for i, legend_settings in enumerate(legend_stack):
    self.legend_obj = QgsLayoutItemLegend(self.layout) # assuming you have layout attribute already
    setattr(self, 'legend_{}'.format(i), QgsLayerTree())
    self.legend_obj.model().setRootGroup(getattr(self, 'legend_{}'.format(i)))

Probably they also should be set before a loop of legend creation like in __init__ or somewhere else. I did it because QGIS was crashing when layout and its legend is created.