Prevent Confluent Kafka from losing messages when producing

The Confluent Kafka library (python version in this case) has a produce method which takes a delivery callback function:

kafka_producer.produce(topic=topic,
                            key=key,
                            value=value,
                            on_delivery=delivery_callback)

This callback is called whether the message was successfully delivered or not:

def delivery_callback(err, msg):

I don't have any retry logic in this function if the message failed, because the docs say it is asynchronous.

Instead every 100 messages or so, I rely on flush() to tell me if any messages weren't successfully produced:

messages_outstanding = kafka_producer.flush()
if messages_outstanding == 0:
   //continue to the next batch of 100
else:
   //produce the batch again

Will flush() account for any messages which failed to produce? (reported as errors in delivery_callback)

In other words, can I be sure flush() won't return zero if any messages failed?

from Prevent Confluent Kafka from losing messages when producing