In the Customizing doc section "All users may only 
view and edit issues, files and messages they create" 
(http://roundup.sourceforge.net/doc-
1.0/customizing.html#all-users-may-only-view-and-edit-
issues-files-and-messages-they-create)
there is some example code that definines a function:

def checker(klass):
    def check(db, userid, itemid, klass=klass):
        return db.getclass(klass).get
(itemid, 'creator') == userid

I think this is missing the return statement that 
returns the internal function (last line below):

def checker(klass):
    def check(db, userid, itemid, klass=klass):
        return db.getclass(klass).get
(itemid, 'creator') == userid
    return check