getnotify() method tries to get a notify from the server (from the SQL statement NOTIFY). If the server returns no notify, the methods returns None. Otherwise, it returns a tuple (couple) (relname, pid), where relname is the name of the notify and pid the process id of the connection that triggered the notify. Remember to do a listen query first otherwise getnotify will always return None.
Read the upstream documentation. For corrections, first check the current manual.