def set_return_handler(self, handler):
self.basic_return_consumer.set_callback(handler)<EOL>
Set ``handler`` as the callback function for undeliverable messages
that were returned by the server.

By default, an exception is raised, which will be handled by
the event loop's exception handler (see :meth:`BaseEventLoop.set_exception_handler <asyncio.BaseEventLoop.set_exception_handler>`).
If ``handler`` is None, this default behaviour is set.

:param callable handler: A function to be called when a message is returned.
    The callback will be passed the undelivered message.