Here's the traceback I got:
===
>>> s.serve_forever()
----------------------------------------
Exception happened during processing of request from ('127.0.0.1', 
54611)
Traceback (most recent call last):
  File "C:\Python30\lib\socketserver.py", line 281, in 
_handle_request_noblock
    self.process_request(request, client_address)
  File "C:\Python30\lib\socketserver.py", line 307, in process_request
    self.finish_request(request, client_address)
  File "C:\Python30\lib\socketserver.py", line 320, in finish_request
    self.RequestHandlerClass(request, client_address, self)
  File "C:\Python30\lib\socketserver.py", line 615, in __init__
    self.finish()
  File "C:\Python30\lib\socketserver.py", line 655, in finish
    if not self.wfile.closed:
AttributeError: 'RequestHandler' object has no attribute 'wfile'
----------------------------------------
===

's' is an instance of socketserver.TCPServer and the handler passed is 
an instance of socketserver.StreamRequestHandler.

I believe this must be a simple typo, so I didn't feel that more 
details is needed at that point.

Let me know if you have any question.

Can you provide a small example of this error?

This problem seems only occurer when a subclass of
socketserver.StreamRequestHandler provide a blank method "setup".

In your log error:

>> AttributeError: 'RequestHandler' object has no attribute 'wfile'

The class 'RequestHandler' is a subclass of
socketserver.StreamRequestHandler?

That's it actually.

I forgot to call base class' setup() method when I redefined it in my 
subclass. When I properly call StreamRequestHandler.setup() on my 
derived class' setup(), everything works fine.

I'm not a socketserver specialist, but I would have expected 
StreamRequestHandler to derive from RequestHandler and not the other 
way round, although it makes little difference to me as a user's point 
of view.

Thanks a lot for pointing my mistake!

Cheers,
-Eric