Please extend the fivetran_connector_sdk.Operations.upsert (table=”three”, data=data) method with an additional optional parameter allowing to pass a default deserializer method similar to json.dumps() default parameter. Currently, when uncommon classes are encountered upsert() fails. The only workaround I could think of is first serialize and then re-deserialize the object.

yield op.upsert(table='table_name', data=json.loads(json.dumps(data, default=lambda o: o.__dict__)))