#5910 [BUG] [Flytekit] Mismatching Type while serializing Union Types in `_make_dataclass_serializable` Issue created by mao3267 Describe the bug Mentioned in the discussion under flyteorg/flytekit#2823. When handling Union types in

_make_dataset_serializable

, we intend to select the first argument in

get_args(python_type)

because

Optional

is equivalent to

Union[ExpectedType, None]

. However, this approach may fail if the type order differs, for example, in

Union[None, ExpectedType]

. An error reproduction example: from dataclasses import dataclass from flytekit import task from flytekit.types.file import FlyteFile from typing import Union @dataclass class InnerDC: ff: Union[None, FlyteFile] @dataclass class DC: inner_dc: InnerDC @task def t_dc() -> DC: return DC(inner_dc=InnerDC(ff="s3://path")) Expected behavior This code snippet should work, meaning that the underlying Union types in dataclasses should be serialized correctly. Additional context to reproduce No response Screenshots No response Are you sure this issue hasn't been raised already? • Yes Have you read the Code of Conduct? • Yes flyteorg/flyte