Exporting multipages Atlas with predefined layout in one PDF file in QGIS 3.x

i have a partial solution...

I know how to do it in QGIS, in atlas tab and then in output check single file export when possible. If you activate this option in your layout and then save you should be able to use it in pyqgis to export atlas in one page (i haven't tried).

If it doesn't work then I try to find the name of the parameter. I have saved two layouts as template with single file export when possible and without single file export when possible.

When i compare the two files, I get only one difference :

<property key="singleFile" value="true"/>

<property key="singleFile" value="false"/>

I tried to find the property singleFile in an atlas object but there is none. There is maybe a solution to do this in QGIS but if you are happy with your code I think the best will be to do the PDF merging process after even in Python if you want.

To dig deeper : I think you could regroup different QgsLayoutExporter() in a list object and then after your loop is done you do one export with exporter.exportToPdf().

I found a partial solution to my question, in that I merge the sperate PDFs in one file. However, this process is very time consuming, and I hope to find a better answer within the Atlas functions.

PdfListe = []
for comp in manager.printLayouts():
    comp.atlas().beginRender()
    comp.atlas().first()
    for i in range(0, comp.atlas().count()):
        settingspdf = QgsLayoutExporter.PdfExportSettings()
        exporter =  QgsLayoutExporter(comp.atlas().layout () )
        exporter.exportToPdf("c:/temp/"+comp.atlas().currentFilename()+".pdf", settingspdf)
        PdfListe.append('C:/temp'+str(comp.atlas().currentFilename())+'.pdf')
        comp.atlas().next()
comp.atlas().endRender()
merger = PdfFileMerger()
for pdf in PdfListe:
    merger.append(PdfFileReader(pdf, 'rb'))
merger.write('c:/temp/atlas.pdf')
merger.close()