Small'n'dirty script to remove the path of the background file in .xopp (xournalpp)

xopp-relativize.py

#! /usr/bin/env python3
# -*- coding: utf8 -*-
#

import sys
import os
import re
import argparse

import gzip

def warn(*what):
    if args.verbose:
        print("WARNING: ", *what, file=sys.stderr)

def error(*what):
    print("ERROR: ", *what, file=sys.stderr)
    sys.exit(1)

# regular expressions
bg_element =   r'<background.*>'
bg_pdf =       r'type="pdf"'
has_pdf_file = r'filename="(.+?)"' # non-greedy!

def replace_with_relative(m):
    a = m.group(1)
    r = os.path.split(m.group(1))[1]
    warn("absolute file name", a)
    warn("relative file name", r)
    return "filename=\"{}\" ".format(r)



# description
parser = argparse.ArgumentParser(description="Remove \
        the absolute path from a background of a .xopp file")
# two  positional (mandatory) argument
parser.add_argument('filein', metavar='filein',
                   help='Input file')
parser.add_argument('fileout', metavar='fileout',
                   help='Output file')
# one flag argument
parser.add_argument("-v", "--verbose", dest='verbose', action="store_true",
                    help="increase output verbosity")
parser.add_argument("-o", "--overwrite", dest='overwrite', action="store_true",
                    help="overwrite output file (stop otherwise)")

args=parser.parse_args()

fin=gzip.open(args.filein, mode="rt", encoding="utf-8")
if (os.path.exists(args.fileout)):
    if not args.overwrite:
        error("File {} exists, aborted (use -o to override)".format(args.fileout))
    else:
        warn("Overwriting output file")
fout=gzip.open(args.fileout, mode="wt", encoding="utf-8")

for rline in fin:
    line=rline.strip()
    if re.match(bg_element, line) and re.search(bg_pdf, line):
        warn("BG found  :", line)
        m = re.search(has_pdf_file, line)
        if m:
            newline = re.sub(has_pdf_file, replace_with_relative, line)
            warn("Substituting with: ", newline)
        else:
            newline = line
        fout.write(newline)
        fout.write("\n")
    else:
        fout.write(rline)

fin.close()
fout.close()

Another way is to use standard unix programs:
zcat absolute.xopp |sed -e 's/filename=\"[^\"]*\//filename=\"/g' |gzip > relative.xopp

To fix all files in a directory (typically exams), I use the following:
ls *.xopp |xargs -I {} sh -c 'zcat {} |sed -e "s/filename=\"[^\"]*\//filename=\"/g" |gzip > {}-~ && mv {}-~ {}'