[Zope] Get userobject from python

[Gitte Wange]

Hello,

I am trying to find a way to find a user object from the user id in a python.
Since I couldn't find anything in the Zope help I looked at the Membership 
tool from CMF.
I looked as if I could try with this code:
user_obj=self.acl_users.getUser(uname)

But get an error saying that the user_obj is None (and therefor it can not 
find the object).

Uname contains the correct Username ...

Anyone who know how to get this object ???

Regards,