I can't recreate that error ...
>>> import twitter
>>> api = twitter.Api(consumer_key='consumer_key',
... consumer_secret='consumer_secret',
access_token_key='access_token',
... access_token_secret='access_token_secret')
>>>
Are you running the latest version of python-twitter?
>>> import twitter
>>> print twitter.__version
0.9-devel
>>>
I haven't followed the development that closely, but I do know that
the oauth is rather recent addition. Twitter switched over to it this
summer. There were some issues relating to it on the google code
message board.
adam