qgis - Iterating over layers and export them as PNG images with PyQGIS in a standalone script?

I would like to export specific layers as a png from a QGIS .qgs map. From the table of contents the script would first export the "boundary, climits, and Div1_Irrig_1956_0" layers as one png. The standalone script would then iterate to export "boundary, climits, and Div1_Irrig_1976_1" layers as the next png and so on. I am working with the script below to begin with but am only getting one png with all of the layers exported.

from PyQt4.QtCore import *
from PyQt4.QtGui import *

from qgis.core import *
from qgis.gui import *
from qgis.utils import iface

import sys, os
import glob

qgs = QgsApplication(sys.argv, True)
qgis_prefix = "C:\\OSGeo4W\\apps\\qgis"
QgsApplication.setPrefixPath(qgis_prefix, True)

qgs.initQgis()

#layers = glob.glob((configData['destination_folder'])+"\\*.shp")
layers = glob.glob(r"E:\IrrigatedLands\FC_qgis\*.shp")

for layer in layers:
    print layer
    irrig = QgsVectorLayer(layer, "testlayer_shp", "ogr")
    print irrig.isValid()
    layerset = []

    QgsMapLayerRegistry.instance().addMapLayer(irrig)
    layerset.append(irrig.id())

    # create image
    imageType = "png"
    pathToFile = "C:\\Users\\James\\Desktop\\"
    name = "render"
    img = QImage(QSize(800, 600), QImage.Format_ARGB32_Premultiplied)

    # set image's background color

    color = QColor(255, 255, 255)
    img.fill(color.rgb())

    # create painter
    p = QPainter()
    p.begin(img)
    p.setRenderHint(QPainter.Antialiasing)

    render = QgsMapRenderer()


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

    # set extent
    rect = QgsRectangle(render.fullExtent())
    rect.scale(1.1)
    render.setExtent(rect)


    # set output size
    render.setOutputSize(img.size(), img.logicalDpiX())

    # do the rendering
    render.render(p)

    p.end()

    img.save(pathToFile + name + "." + imageType ,imageType)

Answer

In order to solve this question, we need to use timers or something that delays the execution of the script, so the canvas can reflect the layer arrangement at the time the map is exported. In other words, if you don't use timers you'll end up with 3 PNG images with the same content because everything will happen too fast.

In the QGIS map, set the map extent you want to export before running the following script in the QGIS Python Console (adjust the path ):

from PyQt4.QtCore import QTimer

fileName = '/tmp/exported' # exported is a prefix for the file names
boundaryLayer = QgsMapLayerRegistry.instance().mapLayersByName('boundary')[0]
climitsLayer = QgsMapLayerRegistry.instance().mapLayersByName('climits')[0]
otherLayers = ['Div1_Irrig_1956_0', 'Div1_Irrig_1956_1', 'Div1_Irrig_1956_2']
count = 0


iface.legendInterface().setLayerVisible(boundaryLayer, True)
iface.legendInterface().setLayerVisible(climitsLayer, True)

def prepareMap(): # Arrange layers
    iface.actionHideAllLayers().trigger() # make all layers invisible
    iface.legendInterface().setLayerVisible(QgsMapLayerRegistry.instance().mapLayersByName( otherLayers[count] )[0], True)
    QTimer.singleShot(1000, exportMap) # Wait a second and export the map

def exportMap(): # Save the map as a PNG
    global count # We need this because we'll modify its value

    iface.mapCanvas().saveAsImage( fileName + "_" + str(count) + ".png" )
    print "Map with layer",count,"exported!"
    if count < len(otherLayers)-1:
        QTimer.singleShot(1000, prepareMap) # Wait a second and prepare next map
    count += 1

prepareMap() # Let's start the fun

After the execution of the script, you'll end up with 3 (different) PNG images in /tmp/.

If you need to iterate over more layers, you just need to add their names to the otherLayers list, the script will do the rest for you.

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EDIT: How to run this as a standalone script (outside of QGIS)?

The following script can be run outside of QGIS. Just make sure you adjust the file paths to your own directory structure and that you use a QGIS prefix that works for your own environment (see this answer for details):

from qgis.core import QgsApplication, QgsMapLayerRegistry, QgsVectorLayer, QgsProject
from qgis.gui import QgsMapCanvas, QgsMapCanvasLayer, QgsLayerTreeMapCanvasBridge
from PyQt4.QtCore import QTimer, QSize

qgisApp = QgsApplication([], True)
qgisApp.setPrefixPath("/usr", True)
qgisApp.initQgis()


# Required variables with your shapefile paths and names
pngsPath = '/tmp/'
boundaryLayer = QgsVectorLayer('/docs/geodata/colombia/colombia_wgs84.shp', 'boundary', 'ogr')
climitsLayer = QgsVectorLayer('/docs/geodata/colombia/colombia-geofabrik/railways.shp', 'climits', 'ogr')
otherLayers = {'Div1_Irrig_1956_0': QgsVectorLayer('/docs/geodata/colombia/colombia-geofabrik/points.shp', 'Div1_Irrig_1956_0', 'ogr'), 
    'Div1_Irrig_1956_1':QgsVectorLayer('/docs/geodata/colombia/colombia-geofabrik/places.shp', 'Div1_Irrig_1956_1', 'ogr'), 
    'Div1_Irrig_1956_2': QgsVectorLayer('/docs/geodata/colombia/colombia-geofabrik/natural.shp', 'Div1_Irrig_1956_2', 'ogr')}
count = 0    


canvas = QgsMapCanvas()
canvas.resize(QSize(500, 500)) # You can adjust this values to alter image dimensions
canvas.show()

# Add layers to map canvas taking the order into account
QgsMapLayerRegistry.instance().addMapLayer( boundaryLayer)
QgsMapLayerRegistry.instance().addMapLayers( otherLayers.values() )
QgsMapLayerRegistry.instance().addMapLayer( climitsLayer )
layerSet = [QgsMapCanvasLayer(climitsLayer)]
layerSet.extend([QgsMapCanvasLayer(l) for l in otherLayers.values() ])

layerSet.append(QgsMapCanvasLayer(boundaryLayer))
canvas.setLayerSet( layerSet )

# Link Layer Tree Root and Canvas
root = QgsProject.instance().layerTreeRoot()
bridge = QgsLayerTreeMapCanvasBridge(root, canvas) 

def prepareMap(): # Arrange layers
    for lyr in otherLayers.values(): # make all layers invisible
        root.findLayer( lyr.id() ).setVisible(0) # Unchecked

    root.findLayer( otherLayers.values()[count].id() ).setVisible(2) # Checked
    canvas.zoomToFullExtent()
    QTimer.singleShot(1000, exportMap) # Wait a second and export the map

def exportMap(): # Save the map as a PNG
    global count # We need this because we'll modify its value
    canvas.saveAsImage( pngsPath + otherLayers.keys()[count] + ".png" )
    print "Map with layer",otherLayers.keys()[count],"exported!"
    if count < len(otherLayers)-1:
        QTimer.singleShot(1000, prepareMap) # Wait a second and prepare next map

    else: # Time to close everything
        qgisApp.exitQgis()
        qgisApp.exit() 
    count += 1

prepareMap() # Let's start the fun
qgisApp.exec_()

Again, if you need to iterate over more layers, just add them to the otherLayers dictionary, the script will do the rest.

The resulting PNG image file names will correspond to your layers.