How to have a default for Pydantic Discriminated Union

[SjoerdTUDelft]

I have a dagster job with many discriminated unions, I want to have simple yaml files without specifying every field to run everything but I run into the issue that for discriminated unions the default always seems to be explicitely given in the yaml even though the default is also present in the pydantic definition.
I looked into the source code of dagster and my impression is that this is not supported, at least looking at the following file I see no default is passed:
dagster/_config/pythonic_config/conversion_utils.py(288)_convert_pydantic_discriminated_union_field
with the following line missing a value for the default:
return Field(config=Selector(fields=dagster_config_field_mapping) })

So my question is whether I can somehow still get the desired effect of using unions without having to specify them in the yaml. And if it is not possible, how difficult does it seem to add this functionality?

An example:

 from typing import Literal, TypeAlias, Union

import dagster as dg
from pydantic import Field


class Cat(dg.Config):
    pet_type: Literal["cat"] = "cat"
    meows: int = 33


class Dog(dg.Config):
    pet_type: Literal["dog"] = "dog"
    barks: float = 3.5


Pets: TypeAlias = Union[Cat, Dog]


class DiscriminatedConfig(dg.Config):
    pet: Pets = Field(discriminator="pet_type", default_factory=Dog)  # Note the default is present


class ExtraClass(dg.Config):
    var: str = "default"


class MainConfig(dg.Config):
    do: DiscriminatedConfig = Field(default_factory=DiscriminatedConfig)
    ex: ExtraClass = Field(default_factory=ExtraClass)


@dg.op
def do_something(context: dg.OpExecutionContext, config: MainConfig):
    context.log.info("config_param: " + str(config.do.pet.pet_type))


default_config = dg.RunConfig(ops={"do_something": MainConfig()})


@dg.job(config=default_config)
def do_it_all_with_default_config() -> None:
    do_something()


do_it_all_with_default_config.execute_in_process()  # WORKS
do_it_all_with_default_config.execute_in_process(  # WORKS
    run_config={"ops": {"do_something": {"config": {"do": {"pet": {"dog": {}}}}}}}
)
do_it_all_with_default_config.execute_in_process(  # ERROR
    run_config={"ops": {"do_something": {"config": {}}}}
) 

Error when running without a value specified in the yaml:

 dagste_git.py:50: in <module>
    do_it_all_with_default_config.execute_in_process(  # WORKS
.venv/lib/python3.11/site-packages/dagster/_core/definitions/job_definition.py:744: in execute_in_process
    return core_execute_in_process(
.venv/lib/python3.11/site-packages/dagster/_core/execution/execute_in_process.py:42: in core_execute_in_process
    execution_plan = create_execution_plan(
.venv/lib/python3.11/site-packages/dagster/_core/execution/api.py:700: in create_execution_plan
    resolved_run_config = ResolvedRunConfig.build(job_def, run_config)
                          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
.venv/lib/python3.11/site-packages/dagster/_core/system_config/objects.py:149: in build
    run_config = run_config_schema.config_mapping.resolve_from_unvalidated_config(
.venv/lib/python3.11/site-packages/dagster/_core/definitions/config.py:89: in resolve_from_unvalidated_config
    raise DagsterInvalidConfigError(
E   dagster._core.errors.DagsterInvalidConfigError: Error in config mapping 
E       Error 1: Missing required config entry "do" at path root:ops:do_something:config. Sample config for missing entry: {'do': {'pet': '<selector>'}}

[Casvharen]

I am also struggling with this issue. If there is a solution out there I would love to know!