Attribute store in field doesn't designed that why (for osv.osv model). If your class inherit from osv.osv I think you can't do what you want to do. Have you consider using osv.osv_abstract?
 

Hi Eric,

Just use the fields.function and make your character field not store into database.

for ex here I am going to make one 'note' field with type character and it will not store into database.

'note' : fields.function( _note_field, string="Note", type="char", size=128, store=False)

def _note_field(self, cr, uid, ids, context={})

    res = {}

    for id in ids:

    # your code for proceessed to get this field information

        res[id] = ' processed data'

    return res

By this way you will get your field with character type and it will not store into database. I hope it will help you.