Getting a layer that is not active with PyQGIS

Question

I want to retrieve attributes of selected objects on a layer with PyQGIS. All example I see is for the active layer:

vlayer=qgis.utils.iface.activeLayer()
selected_bf = vlayer.selectedFeatures()

But, my features are not on the active layer. What is the way to say that vlayer is another layer but not the active layer?

---

I put this:

import processing
from qgis.core import * 
vlayer = processing.getObject('mylayer')
selected_objects = vlayer.selectedFeatures()

as a filter expression in QGIS 2.8 but I can't open QGIS after.

Error: "AttributeError: 'NoneType' object has no attribute 'selectedFeatures'"

Answer 1

You can also reference a layer by it's name:

layer = QgsMapLayerRegistry.instance().mapLayersByName("MY_LAYER_NAME")[0]
selected = layer.selectedFeatures()

if your not sure of the name, you can look for it:

layer=None
for lyr in QgsMapLayerRegistry.instance().mapLayers().values():
    if lyr.name() == "MY_LAYER_NAME":
        layer = lyr
        break

Note for QGIS 3.x: in QGIS3 you should replace QgsMapLayerRegistry by QgsProject (see this answer).

Answer 2

Look at Get a list of layer names using PyQGIS`

You can use a simple Python dictionary

names = [layer.name() for layer in QgsMapLayerRegistry.instance().mapLayers().values()]
layers = dict((name,i) for i, name in enumerate(names))
print layers
{u'layer1': 1, u'layer2': 0, u'layer3': 2, u'layer': 3, ...}

or:

canvas= qgis.utils.iface.mapCanvas()
layers = dict((k.name(),i) for i, k in enumerate(canvas.layers()))
print layers
{u'layer1': 1, u'layer2': 0, u'layer3': 2, u'layer': 3, ...}

And to select the layer:

layer1 = canvas.layer(layers['layer1'])
layer2 = canvas.layer(layers['layer2'])
....
# or
layer1 = canvas.layer(1)
layer2 = canvas.layer(2)
....

But if you add a new layer, you must start over, not with the processing module:

import processing
layer1 = processing.getObject('layer1')
layerx =  processing.getObject('newlayer')