pyqgis - Using QGIS 3.0 API for layout?

I am trying to generate an atlas with python in qgis 3.0. I have :

projectInstance= QgsProject.instance()
projectLayoutManager = projectInstance.layoutManager()

for comp in projectLayoutManager.printLayouts():
    if comp.atlas().enabled():
        comp.atlas().beginRender() 
        nbobj = comp.atlas().updateFeatures()
        comp.atlas().first()
        for i in range(0, comp.atlas().count ()):
            comp.atlas().refreshCurrentFeature ()
            exporter =  QgsLayoutExporter(comp.atlas().layout () )
            pdf_settings = exporter.ImageExportSettings () #dpi?
            exporter.exportToImage (, pdf_settings)

        comp.atlas().endRender() 

But I would like the filename to have some attributes of the atlas feature.

How do I get the value of the attribute?

Answer

You can simplify this script considerably in QGIS 3.0:

projectInstance= QgsProject.instance()
projectLayoutManager = projectInstance.layoutManager()

image_settings = exporter.ImageExportSettings()

image_settings.dpi = 300 # or whatever you want

for comp in projectLayoutManager.printLayouts():
    if comp.atlas().enabled():
        result, error = QgsLayoutExporter.exportToImage(comp.atlas(), 
                            baseFilePath='c:/temp/my_atlas', extension='.png', settings=image_settings)
        if not result == QgsLayoutExporter.Success:
            print(error)

To control the generated filenames, you need to set an expression for the atlas filenames, e.g.:

comp.atlas().setFilenameExpression('"some_attribute" || '_export' )