feat(StreamingWithUnionType): Add support for streaming BaseModel with union Types

[wassim-trabelsi]

Is your feature request related to a problem? Please describe.
I've got a BaseModel with unionTypes and i cannot stream the attribute

from instructor import Partial

class MyResponse:
     value: str | int

Partial[MyResponse]

   1026     modified_args = tuple(_process_generic_arg(arg) for arg in generic_args)
   1028     # Reconstruct the generic type with modified arguments
   1029     tmp_field.annotation = (
-> 1030         generic_base[modified_args] if generic_base else None
   1031     )
   1032 # If the field is a BaseModel, then recursively convert it's
   1033 # attributes to optionals.
   1034 elif isinstance(annotation, type) and issubclass(annotation, BaseModel):
   1035     # Prevent infinite recursion for self-referential models

TypeError: type 'types.UnionType' is not subscriptable

Describe the solution you'd like
A clear and concise description of what you want to happen.

I want to be able to stream MyResponse with the value that has union types.

Describe alternatives you've considered
A clear and concise description of any alternative solutions or features you've considered.

It's not that important, i can workaround and make it work by going toward a more general type like str or list[str]. But i guess for some users, and some usecases, it might be frustrating to not have support for union types.

We might also wait and hope a release of Pydantic that will do the heavy lifting on this part.

Additional context
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[jbbqqf]

Hi @wassim-trabelsi, looking at this from the outside — I think this can likely be closed as already fixed. Quick evidence trail from current main (and the v1.15.0 release):

• instructor/dsl/partial.py:42-45 defines UNION_ORIGINS = (Union, types.UnionType) on Python 3.10+ specifically to catch PEP 604 unions like str | int.
• instructor/dsl/partial.py:241-242 has the explicit handling with the comment "Special handling for Union types (types.UnionType isn't subscriptable)" — which is exactly the TypeError: type 'types.UnionType' is not subscriptable you hit. The branch rewraps via Union[modified_nested_args] so the recursion no longer trips on MyResponse.value: str | int.
• The original fix landed in PR #1207 (commit 934c7281, Nov 2024) and has been part of every release since.
• A regression test for your exact construct was added in PR #2262 (tests/dsl/test_partial.py:217-222):

  class PEP604UnionField(BaseModel):
      value: str | int
  
  def test_partial_streaming_with_pep604_union_field():
      partial = Partial[PEP604UnionField]
      models = list(partial.model_from_chunks(['{"value": ', "1}"]))
      assert models[-1].model_dump() == {"value": 1}

  This is essentially your repro turned into a passing test.

Could you re-try on instructor>=1.15.0 (current is 1.15.2)? If you still see the TypeError on a recent release with a minimal repro (the class MyResponse: in the OP is missing BaseModel inheritance — was that the actual file?), happy to dig further; otherwise this looks safe to close.

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Disclosure: triage drafted with AI assistance (Claude), repro evidence verified locally against main and the v1.15.0 tag.