def disconnect(self, event, cb):
try:<EOL><INDENT>self._callbacks[event].remove(cb)<EOL><DEDENT>except KeyError:<EOL><INDENT>raise ValueError("<STR_LIT>".format(event))<EOL><DEDENT>except ValueError:<EOL><INDENT>raise ValueError("<STR_LIT>".format(event))<EOL><DEDENT>
Disconnect a previously connected callback.

If a callback is connected multiple times, only one connection is
removed.