Exporting two layers with PyQGIS script

Question

I'm trying to export multiple layers in a single image file using PyQgis (QGIS 2.18), however so far only manage to render one layer at a time, not both together. If I try to render both, only the last one added in the script gets plotted.

Here's a sketch showing what I'm trying to achieve:

I tried using canvas.setLayerSet to add both layers to the canvas, assuming the following command can merge multiple layers:

# set the map canvas layer set
canvas.setLayerSet([QgsMapCanvasLayer(layer)])

However, this has no effect, and only the last layer added in the script gets rendered in the PNG file when running:

# Try to add multiple layers to canvas
canvas.setLayerSet([QgsMapCanvasLayer(lyr_perimeter),QgsMapCanvasLayer(lyr_blgs)]) 

What would be the correct way to force multiple layers to be rendered together?

So far I've tried:

• Rendering only one layer at a time --> both work individually (cf. image above)
• Rendering both within QGIS 2.18 GUI --> both can be viewed together (cf. image above)
• Using the simple rendering approach as described in the documentation, however this raises AttributeError: 'QgsVectorLayer' object has no attribute 'getLayerID' when using getLayerID (I assume this function is only available in QGIS 3.0)
• Previously posted a related question, but this question narrows the problem to two layers, specifies new attempts, and provides a (hopefully) clearer minimal script.

Here's the the minimal script being used:

#!/usr/bin/env python2
#############################
    #Import modules for PyQGIS commands
#############################
from qgis.core import *
import os.path
import sys
from qgis.gui import * #QgsMapCanvas, QgsLayerTreeMapCanvasBridge
# from PyQt4.QtCore import QFileInfo, QObject
from PyQt4.QtCore import * #Required for NameError QSize missing, so import all for PNG export
from PyQt4.QtXml import QDomDocument
from PyQt4.QtGui import *   #Required for PNG export (NameError QImage is not defined, see https://gis.stackexchange.com/questions/88660/nameerror-name-qimage-is-not-defined)
from inputs import connection_data #loads from frontend.py the DB connection data


#############################
    #Database connection data to be used in queries below
#############################
#here are input variables for connection#

#############################
    #Various PyQgis specifications
#############################
gui_flag = True #Pass true to indicate using a GUI for a custom application (as opposed to standalone script)
app = QgsApplication(sys.argv, gui_flag)
app.initQgis()
# Set output DPI
dpi = 300


#############################
    #Define paths for input and output files
#############################
my_path = os.path.abspath(os.path.dirname(__file__)) #gets absolute path to script
root_path = os.path.join(my_path, '../../media/maps/qgis') #goes to relative upper dir
project_path = '/prjs/autoexport_empty.qgs' 
result_path = '../../media/maps/tmp'
result_path = os.path.join(my_path, '../../media/maps/tmp')


#############################
    #Main function inputs to be passed
#############################
scenario_id = str(sys.argv[1])          #1315     
cityobjectgroup_id = str(sys.argv[2])   #27787


def make_pdf():
    canvas = QgsMapCanvas()

    # Load our project
    QgsProject.instance().read(QFileInfo(root_path + project_path))

    # Load layers here if they are not already in the project
    #for layer in QgsMapLayerRegistry.instance().mapLayers().values():
        #if layer.name() == 'data':
            #lyr = layer
            #break
    #lyr.setSubsetString(filter_expression)
    uri = QgsDataSourceURI()
        # set host name, port, database name, username and password
        uri.setConnection("{}".format(host), "{}".format(port), "{}".format(dbname), "{}".format(user), "{}".format(password))


##########################################################
        #Add first layer to image
##########################################################
        # PERIMETER       
        # Set database schema, table name, geometry column and optionally a subset (WHERE clause)
        uri.setDataSource(schema_name2, "geo_girec", "geom", "numero = '2800152'") 

        # Define primary key of view (https://gis.stackexchange.com/questions/151686/selecting-column-like-primary-key-in-view-from-postgis-using-pyqgis)
        uri.setKeyColumn('numero')

        # Load layer to PyQgis environment
        print "loading PERIMETER layer"
        lyr_perimeter = QgsVectorLayer(uri.uri(False), "perimeter", "postgres")

        # Check if layer was correctly loaded to PyQgis and if so, process its style
        if not lyr_perimeter.isValid():
            print "Layer 'lyr_perimeter' failed to load! Probably not existent."
        else:
            # Add layer to the registry                    
            QgsMapLayerRegistry.instance().addMapLayer(lyr_perimeter)
            print "QgsMapLayerRegistry correctly added map layer lyr_perimeter"

            # Set canvas extent  
            canvas.setExtent(lyr_perimeter .extent())

            # Load style file corresponding to the layer
            lyr_perimeter.loadNamedStyle(root_path + '/layerStyles/perimeter_style' + '.qml')
            print "Checking root path:"
            print root_path + '/layerStyles/perimeter_style' + '.qml'

            lyr_perimeter.triggerRepaint()


##########################################################
        #Add second layer to image
##########################################################
        # BUILDINGS        
        # Set database schema, table name, geometry column and optionally a subset (WHERE clause)
        uri.setDataSource(schema_name, "building_view", "geometry", "scenario_id = " + scenario_id + " AND cityobjectgroup_id = " + cityobjectgroup_id)

        # Define primary key of view (https://gis.stackexchange.com/questions/151686/selecting-column-like-primary-key-in-view-from-postgis-using-pyqgis)
        uri.setKeyColumn('id,scenario_id,cityobjectgroup_id')

        # Load layer to PyQgis environment        
        print "loading BUILDING_VIEW layer"
        lyr_blgs = QgsVectorLayer(uri.uri(False), "buildings", "postgres")

        # Check if layer was correctly loaded to PyQgis and if so, process its style        
        if not lyr_blgs.isValid():
            print "Layer 'building_view' failed to load! Probably not existent."
        else:
            # Add layer to the registry
            QgsMapLayerRegistry.instance().addMapLayer(lyr_blgs)
            print "QgsMapLayerRegistry correctly added map layer lyr_blgs"

            # Set canvas extent  
            canvas.setExtent(lyr_blgs.extent()) 

            # Try to add multiple layers to canvas --> this has no effect! only lyr_blgs gets printed to PNG
            canvas.setLayerSet([QgsMapCanvasLayer(lyr_perimeter),QgsMapCanvasLayer(lyr_blgs)])      

            # Load style file corresponding to the layer
            lyr_blgs.loadNamedStyle(root_path + '/layerStyles/citygml/2.5d-height_label-floorstotal' + '.qml') #previously from sty object
            print "Checking root path:"
            print root_path + '/layerStyles/citygml/2.5d-height_label-floorstotal' + '.qml'

            lyr_blgs.triggerRepaint()


##########################################################
        #Render map to PNG
##########################################################

            # Bridge used in standalone script: http://docs.qgis.org/testing/en/docs/pyqgis_developer_cookbook/loadproject.html; has to be created before further layers are loaded
            bridge = QgsLayerTreeMapCanvasBridge(
                    QgsProject.instance().layerTreeRoot(), canvas)                     
            bridge.setCanvasLayers()

            # Load print composer template
            template_path = root_path + '/printComposers/autoexport_Palettes' + '.qpt' #previously from sty object sty['qpt']
            template_file = file(template_path)
            template_content = template_file.read()
            template_file.close()
            print "Finished loading print composer template!"
            document = QDomDocument()

            document.setContent(template_content)
            composition = QgsComposition(canvas.mapSettings())    

            # You can use this to replace any string like this [key]
            # in the template with a new value. e.g. to replace
            # [date] pass a map like this {'date': '1 Jan 2012'}
            # substitution_map = {
            #         'title': 'the title of my map'}
            composition.loadFromTemplate(document, {})

            # You must set the id in the print composer template
            map_item = composition.getComposerItemById("myMap")     # to verify if this is the right idea, otherwise try also 'main' or 'map 0'                  

            map_item.setMapCanvas(canvas)
            map_item.zoomToExtent(canvas.extent())                  # or use directly extent of building_view "layer"
            # You must set the id in the template
            #legend_item = composition.getComposerItemById('Legend')
            #legend_item.updateLegend()

            composition.refreshItems()

            # Export to png
            dpmm = dpi / 25.4
            width = int(dpmm * composition.paperWidth())
            height = int(dpmm * composition.paperHeight())

            # create output image and initialize it
            image = QImage(QSize(width, height), QImage.Format_ARGB32)
            image.setDotsPerMeterX(dpmm * 1000)
            image.setDotsPerMeterY(dpmm * 1000)
            image.fill(0)

            # render the composition
            imagePainter = QPainter(image)
            composition.renderPage(imagePainter, 0)
            imagePainter.end()


            # Save image as PNG file
            image.save(result_path + '/' + "myPNG_" + scenario_id + '.png', "png")


    QgsProject.instance().clear()

Answer 1

Following the documentation, you will only have to change layer.getLayerId() to layer.id():

# set layer set
lst = [layer.id()]  # add ID of every layer
render.setLayerSet(lst)

I figured this out by "accident", but the id() function is described here, as for the getLayerId() function, I could not find any docs. I proposed a change to the "simple rendering" documentation to fix this.