code: def _get_notmuch_message(self, mid):
          
          mode = Database.MODE.READ_ONLY
          db = Database(path=self.path, mode=mode)
          try:
              return db.find_message(mid)
          except:
              errmsg = 'no message with id %s exists!' % mid
              raise NonexistantObjectError(errmsg)
docs: returns :class:`notmuch.database.Message` with given id