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
Add any other context or screenshots about the feature request here.

[xXMrNidaXx]

Union types in streaming is tricky because partial validation needs to handle type discrimination.

The issue: Python 3.10+ UnionType (str | int) isn't directly subscriptable like Union[str, int].

Quick fix in the Partial logic:

from typing import Union, get_origin, get_args
import types

def _process_generic_arg(arg):
    # Handle UnionType (str | int) from Python 3.10+
    if isinstance(arg, types.UnionType):
        # Convert to typing.Union
        return Union[get_args(arg)]
    return arg

Or in the annotation check:

if isinstance(annotation, types.UnionType):
    # Treat as Optional during streaming
    union_args = get_args(annotation)
    annotation = Union[tuple(union_args) + (type(None),)]

Workaround for now:

from typing import Union

class MyResponse(BaseModel):
    value: Union[str, int]  # Use typing.Union instead of |

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