[Zope] xmlrpc calls require string results?

[Dan Pierson]

I'm using xmlrpclib from a standalone Python script to post articles to
Squishdot.  It seems that calls to methods that return strings work fine,
while calls to other methods fail with tracebacks like the following:

Traceback (innermost last):
  File "./postToControlCom.py", line 77, in ?
    postMessage(msg)
  File "./postToControlCom.py", line 68, in postMessage
    summary, subject, "Automation List")
  File "./xmlrpclib.py", line 547, in __call__
    return self.__send(self.__name, args)
  File "./xmlrpclib.py", line 630, in __request
    request
  File "./xmlrpclib.py", line 582, in request
    headers
xmlrpclib.ProtocolError: <ProtocolError for silver.control.com/zope/: 500
Internal Server Error>

This sort of thing seems to happen no matter what the non-string return type
is.  In the case above, the difference between the following code (which
works) and the failing code is exactly the rather bogus last line: "return
'Done'".
The code is in zope as an External Method (since I can't use xmlrpc to do
the same thing from my script).

def postSquishdotReply(self, title, author, body, email='', notify='',
file='',
                       summary='', subject='', dept='',
                       REQUEST=None, RESPONSE=None):
    results = self.__call__({'title':title, 'subject':subject,
'op':'articles'})
    if len(results):
        # If there are multiple Articles with the same title and subject,
        # just use the first one.
        article = results[0]
        article.addPosting(title, author, body, email, notify, file,
                           REQUEST, RESPONSE)
    else:
        self.addPosting(title, author, body, email, notify, file,     
                        summary, subject, dept, REQUEST, RESPONSE)

    # It appears that calls via xmlrpclib throw:
    #   xmlrpclib.ProtocolError:
    #   <ProtocolError for silver.control.com/zope/: 500 Internal Server
Error>
    # if a method called by it doesn't return a string.
    # This is bogus, but:
    return 'Done'

BTW: all of this appears to me to be true whether or not Squishdot is
involved -- I have tested with target methods that are not part of it.

What am I doing wrong here?

Dan Pierson, Control Technology Corporation, <dan@control.com>