Rendering a map with Python works inside QGIS, but not in standalone script

Question

I have an existing QGIS project and I want to write a standalone Python script that outputs a png file of the map. I work on Windows 10. This is the code that I use (basically taken from the PyQGIS cookbook):

import os
from qgis.core import *
from PyQt5 import *
from PyQt5.QtSvg import *
from PyQt5.Qt import *

app = QgsApplication([], True) 
app.setPrefixPath(r"C:/Program Files/QGIS 3.10/apps/qgis", True) 
app.initQgis() 

project = QgsProject.instance() 
project.read("filename.qgz")
layer = project.mapLayersByName("myLayer")[0]

options = QgsMapSettings()
options.setLayers([layer])
options.setBackgroundColor(QColor(255, 255, 255))
options.setOutputSize(QSize(800, 600))
options.setExtent(layer.extent())
render = QgsMapRendererParallelJob(options)
image_location = os.path.join(os.getcwd(), "render.png")

def finished():
    img = render.renderedImage()
    img.save(image_location, "png")
    print("saved")

render.finished.connect(finished)
render.start()

Executing these commands line by line in the Python console inside QGIS works perfectly fine and it outputs a png file with the map in a fraction of a second. However running this from the Python interpreter, it does not output anything. When calling render.isActive(), it returns True.

Is this a bug? Or do I have to do things differently when I want to render a map in a standalone script?

Answer 1

Add render.waitForFinished() after render.start()

def finished():
    img = render.renderedImage()
    img.save(image_location, "png")

render.finished.connect(finished)
render.start()
render.waitForFinished()

Or

render.start()
render.waitForFinished()
img = render.renderedImage()
img.save(image_location, "png")