Python producers ignore inherited Avro content type; Kafka binary marshaller always emits JSON #670

Reproducer

Toolchain: xrcg 0.11.0, avrotize 3.9.1, Python.

Generate the public Bluesky contract with an inherited Avro message variant:

{
  "messageid": "Bluesky.Feed.Post",
  "basemessage": "/messagegroups/BlueskyFirehose/messages/Bluesky.Feed.Post",
  "dataschemaformat": "Avro",
  "dataschemauri": "#/schemagroups/BlueskyFirehose.avro/schemas/Bluesky.Feed.Post",
  "envelopemetadata": {
    "datacontenttype": {
      "type": "string",
      "value": "application/vnd.apache.org.avro+avro"
    }
  }
}

The base message uses CloudEvents/1.0, with type, source and subject metadata. The referenced schema is an Avro record. Associate the variant group with a Kafka endpoint and generate:

xrcg generate --style kafkaproducer --language py --projectname bluesky_producer --definitions bluesky.xreg.json --output generated

The generated variant class contains the declared content type in its attributes, but its send method has:

content_type: str = "application/json"
# ...
attributes["datacontenttype"] = content_type

Its binary marshaller ignores content_type:

payload = data.to_byte_array("application/json")

Observed behavior

Executed generated BlueskyFirehoseAvroBinaryEventProducer.send_bluesky_feed_post in CloudEvents binary mode using a generated Post and a recording Kafka producer:

This affects serialization, not only metadata. An explicit argument does not fix Kafka output. The structured branch also always uses json.loads(x.to_json()), rather than serializing binary Avro and representing it as CloudEvents JSON data_base64.

The inherited classes and their six message methods are generated, and all generated Python compiles. Inheritance itself resolves the declared metadata; the producer template then overrides it.

AMQP generation from the same contract also defaults the binary-Avro variant's content_type to application/json. No AMQP wire claim is made here.

Expected

• Default the payload content type from effective inherited CloudEvents metadata.
• Select the payload serializer using that content type.
• Binary Avro must produce Avro bytes, including when nested in a structured CloudEvent.
• Preserve the JSON variant's application/json encoding even when its schema is Avro.

No generated-code edits or test workarounds were applied.