class MemberListing(grok.View):

    def fieldNames(self):
        return (f for f in IUser)

    def members(self):
        pau = component.getUtility(IAuthentication)
        principals = pau['principals']
        roster = [ ]
        for id in sorted(principals.keys()):
            print id
            print principals[id]
            user = IUser(principals[id])        # <----- this is where the code is breaking
            fields = {}
            for field in IUser:
                fields[field] = getattr(user, field)
            roster.append(fields)
        return roster