Set user defined variables/custom properties for LibreOffice document via CLI

So there are numerous questions how to convert ODT to PDF via command line.
I successfully used

soffice --headless --convert-to pdf:"writer_pdf_Export:SelectPdfVersion=1" --outdir outdir testDocument.odt

However my document contains user defined variables that are injected into the text using fields. I would like to set such variable value via command line so that at PDF generation the correct text is contained. I have done similar stuff on MS Word, but how would this work in Writer?

And if it helps, I can move to custom properties instead of variables - but I would still have to set them on the CLI.

What failed for me was

soffice --headless --convert-to "odt:{\"Version\":{\"type\":\"string\",\"value\":\"1.0\"}}" ReadMe.odt 
convert /home/queeg/ReadMe.odt -> /home/queeg/ReadMe.odt using filter : {"Version":{"type":"string","value":"1.0"}}
Overwriting: /home/queeg/ReadMe.odt
Error: Please verify input parameters... (SfxBaseModel::impl_store <file:///home/queeg/ReadMe.odt> failed: 0x81a(Error Area:Io Class:Parameter Code:26) ./sfx2/source/doc/sfxbasemodel.cxx:3202 ./sfx2/source/doc/sfxbasemodel.cxx:1775)

This command did not error out but also seemed to have no effect on my document. It did neither set the custom property Version nor the custom variable Version:

soffice --headless --convert-to "pdf:draw_pdf_Export:{\“Version\”:{\“type\”:\“string\”,\“value\”:\“draft\”}}" ReadMe.odt 
convert /home/queeg/ReadMe.odt -> /home/queeg/ReadMe.pdf using filter : draw_pdf_Export:{\“Version\”:{\“type\”:\“string\”,\“value\”:\“draft\”}}
Overwriting: /home/queeg/ReadMe.pdf

So I found a solution based on https://python-ooo-dev-tools.readthedocs.io/en/latest/index.html that I want to share with others and my future self.

It consists of a python script that can load a document, set user-defined properties and save the document back as ODT, or PDF or any other supported output format.

For installation on a fresh Ubuntu system, run

sudo apt-get install pip3
sudo pip install ooo-dev-tools

Then install this script:

#!/bin/python3

import sys
from ooodev.utils.lo import Lo
from ooodev.utils.info import Info
from com.sun.star.beans import XPropertySet
from com.sun.star.beans import XPropertyAccess

doc = None

def main() -> int:
  print (sys.argv)

  with Lo.Loader(Lo.ConnectSocket(headless=True)) as loader:
    i = 0
    while i < len(sys.argv):
      arg = sys.argv[i]
      match arg:
        case "--load":
          i=i+1
          docname = sys.argv[i]
          print("loading " + docname)
          doc = Lo.open_doc(fnm=docname, loader=loader)
        case "--set":
          i=i+1
          val = sys.argv[i]
          items = val.split("=")
          name = items[0]
          value = items[1]
          print("setting " + name + "=>" + value)
          
          user_props = Info.get_user_defined_props(doc) # XPropertyContainer
          ps = Lo.qi(XPropertySet, user_props, True)
          
          try:
            ps.setPropertyValue(name, value)
          except Exception:
            pa = Lo.qi(XPropertyAccess, user_props, True)
            names = []
            for propertyvalue in pa.getPropertyValues():
              names.append(propertyvalue.Name)
            print ("Cannot set property '" + name + "'. Known properties are ", names)
            return 1
        case "--save":
          i=i+1
          docname = sys.argv[i]
          print("saving " + docname)
          Lo.save_doc(doc=doc, fnm=docname)
        case _:
          print (i, sys.argv[i])
      i=i+1
    return 0
  
if __name__ == "__main__":
    raise SystemExit(main())

Finally run the code like this:

convert.py --load mydoc.odt --set propname=propvalue -save mydoc.pdf

Be aware you can use all the options multiple times and they are evaluated from left to right.
That means you can load a document, set multiple properties, save it into two output documents, load the next…