my_layer = QgsVectorLayer('Polygon', 'My layer', 'memory')
QgsMapLayerRegistry.instance().addMapLayer(my_layer, False)

# I want to place the layer at a specified position in the layer tree.
root_node = QgsProject.instance().layerTreeRoot()
root_node.insertLayer(3, my_layer)

label = QgsPalLayerSettings()
label.readFromLayer(my_layer)
label.enabled = True
label.fieldName = '$area'
label.writeToLayer(my_layer)

my_layer.setCustomProperty('labeling', 'pal')
my_layer.setCustomProperty('labeling/enabled', 'true')
my_layer.setCustomProperty('labeling/fieldName', '$area')

my_layer.triggerRepaint()
iface.mapCanvas().refresh()

my_layer.setCustomProperty("labeling/drawLabels",  "True")

label.drawLabels = True

label.isExpression = True

label.drawLabels = True