Getting name of layer's parent group

Question

I'm trying to use pyqgis to retrieve the name of the group in which a layer with a specified name is located. My function code is as follows:

def runA4(self,inputRasterName,outputFilename,outputLayerName):
    root = QgsProject.instance().layerTreeRoot()
    print "root:"
    print root

    demRaster = root.findLayer(inputRasterName)
    print "demRaster:"
    print demRaster

    parentGroup = demRaster.parent()
    print "parentGroup:"
    print parentGroup

This gives the following output in the Python console:

root:
<qgis._core.QgsLayerTreeGroup object at 0x0000000025287730>
demRaster:
None

And the following in the "Python error" log messages panel:

2017-10-17T16:42:40 1   Traceback (most recent call last):
    File "C:/Users/Gareth/.qgis2/python/plugins\myPlugin\myPlugin.py", line 1749, in runA4D
        self.runA4(inputRasterName,outputFilename,outputLayerName)
    File "C:/Users/Gareth/.qgis2/python/plugins\myPlugin\myPlugin.py", line 1761, in runA4
        parentGroup = demRaster.parent()
    AttributeError: 'NoneType' object has no attribute 'parent'

So there's something going wrong when I try to grab demRaster using inputRasterName, which is a string containing the name of the layer as listed in the layer tree. What am I doing wrong here?

I can't help but think there must be a better way to do this - it seems like it should be a very simple operation!

Answer 1

As the documentation says, the findLayer() method need a layer Id (https://qgis.org/api/classQgsLayerTreeGroup.html) and not the layer name.

You can get the layer Id from the name this way:

QgsMapLayerRegistry.instance().mapLayersByName('layerName')[0].id()

So your function must be something like this :

def runA4(self,inputRasterName,outputFilename,outputLayerName):
    root = QgsProject.instance().layerTreeRoot()
    print "root:"
    print root

    demRaster = root.findLayer(QgsMapLayerRegistry.instance().mapLayersByName(inputRasterName)[0].id())
    print "demRaster:"
    print demRaster

    parentGroup = demRaster.parent()
    print "parentGroup:"
    print parentGroup

You can also check if everything is right by some print statement with the name() method that is callable on every objects of the QgsLayerTreeRoot.

Answer 2

you can create a simple function for obtain the parent group name,for example:

from qgis.core import *

_layerreg = QgsMapLayerRegistry.instance()

def LayerParentGroupFromName(name):
    root = QgsProject.instance().layerTreeRoot()
    layers = QgsMapLayerRegistry.instance().mapLayersByName(name)
    for layer in layers:
        root_layer = root.findLayer(layer.id())
        if root_layer:
            print root_layer.parent().name ()

And for usage:

LayerParentGroupFromName("airports")