How to Change the Active Layer in QGIS Console – Complete Tutorial

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I want my script to perform actions on a particular layer. Online advice I've found so far suggests using the iface method "setActiveLayer()". Makes sense based on the name alone.

Scenario: In the Layers window I have a layer named "Province" highlighted (active). Then I run the following code:

>>> vl = QgsMapLayerRegistry.instance().mapLayer( 'na_roads_Prov' )
>>> iface.setActiveLayer(vl)

Console Output = False

As far as I understand executing this code in the console should move the highlighted "active layer" from "Province" to 'na_roads_Prov'. Yes? Instead the "Province" layer remains highlighted as active, and the console output says "False". Any ideas? Am I not referring to the layer correctly?

Best Answer

For QGIS3 this functionality seems to be mapped to QgsProject now:

# I get the layer object by the name 'na_roads_Prov'
roads = QgsProject.instance().mapLayersByName('na_roads_Prov')[0]
# I select the layer object
iface.setActiveLayer(roads)

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If you look at the API documentation for QgsVectorDataProvider you'll see that the select method returns void (see http://qgis.org/api/1.8/classQgsVectorDataProvider.html). This is why A is of NoneType in Python.

To access the features you need to iterate over the selection set after your select statement. Here is an example using a shapefile of cities, zoomed to a small area:

iface = qgis.utils.iface
canvas = iface.mapCanvas()
layer = canvas.layer(0)
provider = layer.dataProvider()
provider.select(provider.attributeIndexes(), canvas.extent())
feature = QgsFeature()
while provider.nextFeature(feature):
    attributes = feature.attributeMap()
    print attributes[0].toString()

This results in:

... 
Seward
Anchorage
>>>

See Iterating over Vector Layer in the PyQGIS Cookbook: http://www.qgis.org/pyqgis-cookbook/vector.html

[GIS] PyQgis question: How to change or set active layer

You can set the active layer using the qgis.utils.iface object (i.e. QgisInterface):

qgis.utils.iface.setActiveLayer(QgsMapLayer)  # returns bool
# or
qgis.utils.iface.legendInterface().setCurrentLayer(QgsMapLayer)  # returns bool

Clearing the selection, or the active layer (underlined QTreeWidget item in QGIS 2.0), in the legend is a bit more involved:

from PyQt4.QtCore import *
from PyQt4.QtGui import *
lg = qgis.utils.iface.mainWindow().findChild(QTreeWidget, 'theMapLegend')
lg.selectionModel().clear()  # clear just selection
lg.setCurrentItem(None)  # clear selection and active layer

There are three ways to retrieve the current layers, relative to what you may want to do:

~ via QList< QgsMapLayer * > QgsLegendInterface::layers()

qgis.utils.iface.legendInterface().layers()  # returns [layer, ...]

~ via QList< QgsMapLayer * > QgsMapCanvas::layers():

qgis.utils.iface.mapCanvas().layers()  # returns [layer, ...]

~ via const QMap< QString, QgsMapLayer * > & QgsMapLayerRegistry::mapLayers():

QgsMapLayerRegistry.instance().mapLayers()  # returns {'layer_id': layer, ...}

Reference these answers for information about the differences in those layer collections.

Best Answer

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