writing xml to a buffer (using libxml)

Salut,

is there any way to write a xml tree to a buffer. I tried 

report = StringIO.StringIO()
reportDoc.saveFormatFileTo(report, 'ISO-8859-1', 1)

but that doesn't work. The error code is

Error Type: AttributeError
Error Value: _o

Any ideas? Thanks in advance!

Jens.