Internal Parser Exception when serializing empty schema/ model with avro-json

[ciarandorney]

Describe the bug
Serializing empty schema/model using avro-json results in a fastavro Internal Parser Exception

To Reproduce

from uuid import UUID
from dataclasses_avroschema.pydantic import main as pydantic

class BaseMessage(pydantic.AvroBaseModel):

    pass

class Message(BaseMessage):

    message_uuid: UUID

Expected behavior
The should serialize without error

Actual behavior
avro serialization:

>>> BaseMessage.fake().serialize()
b''
>>> Message.fake().serialize()
b'H4253ac66-2d8b-4470-ba92-25b0b97540d5

avro-json serialization:

>>> Message.fake().serialize(serialization_type="avro-json")
b'{"message_uuid": "4b82449d-4b8f-45e0-a991-e70d8643d20d"}'
>>> BaseMessage.fake().serialize(serialization_type="avro-json")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/ciarandorney/Library/Caches/pypoetry/virtualenvs/py3.10/lib/python3.10/site-packages/dataclasses_avroschema/pydantic/main.py", line 69, in serialize
    return serialization.serialize(
  File "/Users/ciarandorney/Library/Caches/pypoetry/virtualenvs/py3.10/lib/python3.10/site-packages/dataclasses_avroschema/serialization.py", line 28, in serialize
    fastavro.json_writer(file_like_output, schema, [payload])
  File "/Users/ciarandorney/Library/Caches/pypoetry/virtualenvs/py3.10/lib/python3.10/site-packages/fastavro/json_write.py", line 79, in json_writer
    return writer(
  File "/Users/ciarandorney/Library/Caches/pypoetry/virtualenvs/py3.10/lib/python3.10/site-packages/fastavro/_write_py.py", line 739, in writer
    output.flush()
  File "/Users/ciarandorney/Library/Caches/pypoetry/virtualenvs/py3.10/lib/python3.10/site-packages/fastavro/_write_py.py", line 590, in flush
    self.encoder.flush()
  File "/Users/ciarandorney/Library/Caches/pypoetry/virtualenvs/py3.10/lib/python3.10/site-packages/fastavro/io/json_encoder.py", line 96, in flush
    self._parser.flush()
  File "/Users/ciarandorney/Library/Caches/pypoetry/virtualenvs/py3.10/lib/python3.10/site-packages/fastavro/io/parser.py", line 167, in flush
    raise Exception(f"Internal Parser Exception: {top}")
Exception: Internal Parser Exception: <fastavro.io.symbols.Sequence object at 0x1076a9c00>

[marcosschroh]

Yes. The problem is that BaseMessage does not have any fields, which is expected:

print(BaseMessage.avro_schema())

# {"type": "record", "name": "BaseMessage", "fields": []}

Then is it nos possible to serialize an empty payload with a avro schema that does not have any fields. I think the error makes sense.

[ddevlin]

Hi @marcosschroh, we weren't sure if this was the expected behaviour as it's inconsistent between serialisation types, i.e. it's an error when serialising with the avro-json serialisation type, but with avro serialisation there's no error.

[marcosschroh]

Hi,

Even though it does not have too mush sense to have an avro schema without fields I agree that the behaviour is inconsistent. The output should be b'{}' for avro-json when the schema has not fields

[ciarandorney]

Even though it does not have too mush sense to have an avro schema without fields I agree that the behaviour is inconsistent. The output should be b'{}' for avro-json when the schema has not fields

Hi @marcosschroh, does it make sense to raise this issue over at fastavro? It could be patched here but that would involve checking for nested schemas with no fields on serialization.

Another example that produces the same internal parser error:

Avro schema:
{'type': 'record', 'name': 'NestedMessage', 'fields': [{'name': 'uuid', 'type': {'type': 'string', 'logicalType': 'uuid'}, 'default': '13673bf0-37ef-45bf-bf96-1d633ff90275'}, {'name': 'timestamp', 'type': {'type': 'long', 'logicalType': 'timestamp-millis'}, 'default': 1700052483304}, {'name': 'nested', 'type': {'type': 'record', 'name': 'Nested', 'fields': []}, 'default': {}}]} 

Payload:
{'uuid': UUID('16aca70b-37ff-46e5-b055-20c8bbc3ea08'), 'timestamp': datetime.datetime(2012, 8, 20, 19, 19, 19, tzinfo=datetime.timezone.utc), 'nested': {}}

[marcosschroh]

I think we should create an issue in fastavro and in the meantime we can patch it here

[marcosschroh]

Does someone of you want to create the issue in fastavro? I can do it as well, no problem

[ciarandorney]

Does someone of you want to create the issue in fastavro? I can do it as well, no problem

It's on my to-do 😅 If you want to do it go for it 😄

[ciarandorney]

fastavro/fastavro#732