Thanks Arjen,
def q(v):
return b'"' + v.replace(b'"', b'""') + b'"'
return b','.join(q(f) for f in fields) + b'\n'
In the end I also had some other problems with the XML (namespaces), so I used:
etree.tostring(element, method='c14n', exclusive=True)
This helped. My code is now doing it's job.
Regards
Johann