[QGIS-Developer] Unable to export map with PyQGIS 3

[Dimitris Kar]

I am trying to learn and work with PyQGIS 3.

It is a very challenging task cause documentation is really [limited and
out of date][1].

What I am trying to do is to:

 1. create a project
 2. add a layer
 3. save the project
 4. export it as a png

I have done the first 3 steps (see code below).

    import sys
    import qgis
    from qgis.core import QgsVectorLayer
    from qgis.core import QgsApplication
    from qgis.core import QgsProject
    from qgis.core import QgsMapRendererJob, QgsMapSettings
    from qgis.gui import QgsMapCanvas
    from PyQt5.QtGui import QImage, QColor, QPainter
    from PyQt5.QtCore import QSize

    # PYTHON APPLICATIONS

    # prefix path: the location where qgis is installed in the system
    # the easiest way to find the prefix path to run the following commaand
from the python console of qgis: QgsApplication.prefixPath()

    #1. CREATE THE APPLICATION
    QgsApplication.setPrefixPath('/usr', True)
    # second parameter into false disables the gui
    qgs = QgsApplication([], False)
    #load providers
    qgs.initQgis()

    #2. CREATE THE PROJECT
    # create project
    project_name = '/home/dkar/workspaces/qgis/data/test_project_2.qgz'
    project = QgsProject.instance()
    new_project = project.write(project_name)

    #3. READ/OPEN THE PROJECT
    # modify the project e.g. adding more layers and save it.
    new_project = project.read(project.fileName())


    #4. ADD A VECTORLAYER
    # add a layer in the QgsProviderRegistry
    layer=QgsVectorLayer("/home/dkar/workspaces/qgis/data/pakistan_1.shp",
"pakistan", "ogr")

    if not layer.isValid():
        print("Layer failed")

    #5. ADD LAYER TO THE REGISTRY
    project.addMapLayer(layer, True)
    project.write('/home/dkar/workspaces/qgis/data/test_project_2.qgz')

    #6. SAVE THE PROJECT
    all_layers = QgsProject.instance().mapLayers()


    img = QImage(QSize (800, 600), QImage.Format_ARGB32_Premultiplied)
    # set image's backgroud color
    color = QColor(255, 255, 255)
    img.fill(color.rgb())

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

    # THIS IS WHERE THE ISSUE APPEARS
    # THIS IS WHERE THE ISSUE APPEARS
    render = QgsMapRendererJob() # THIS IS WHERE THE ISSUE APPEARS

    # set layer set (am I using here the prohect?)
    lst = [layer.id()] # cause "layer" is the name of the variable
    render.setLayerSet(lst)

    # to remove the provider and layer registries from memory
    qgs.exitQgis()



But when I try to do the last part, following the instructions from the
existing documentation. I get issues because the specific function
(`QgsMapRenderer`) doesn't exist anymore in QGIS 3.

I am not sure how to solve the issue.

I see in the documentation that this function:

    QgsMapRenderer

was replaced by

    QgsMapRendererJob

But then when I try to import and use it in the code as:

    from qgis.core import QgsMapRendererJob, QgsMapSettings
        render = QgsMapRendererJob()

I get an error message:

    TypeError: qgis._core.QgsMapRendererJob represents a C++ abstract class
and cannot be instantiated

Any idea how to proceed with this?

  [1]:
https://docs.qgis.org/testing/pdf/en/QGIS-testing-PyQGISDeveloperCookbook-en.pdf
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