fix: allow union types in streaming_action.pydantic stream_type parameter

[mvanhorn]

Python 3.10+ union syntax (MyModel1 | MyModel2) creates a types.UnionType, which the stream_type parameter rejected because it only accepted Type[BaseModel] or Type[dict].

Added types.UnionType to the accepted types in three places:

• burr/integrations/pydantic.py: PartialType alias (line 272)
• burr/integrations/pydantic.py: _validate_and_extract_signature_types_streaming parameter (line 293)
• burr/core/action.py: streaming_action.pydantic parameter (line 1514)

Before:

stream_type=MyModel1 | MyModel2  # TypeError

After:

stream_type=MyModel1 | MyModel2  # works

Both files already import types, so no new imports needed. types.UnionType is available in Python 3.10+; on 3.9, the | syntax isn't available anyway so the added type is inert.

Fixes #607

This contribution was developed with AI assistance (Claude Code).

[skrawcz]

Thanks! Can you validate that pyright doesn't complain? That's the litmus test here.

[mvanhorn]

Ran pyright (1.1.408) against the two changed files and compared to main:

• burr/core/action.py: 38 errors on both main and this branch (0 new)
• burr/integrations/pydantic.py: 11 errors on both main and this branch (0 new)

No new pyright issues introduced by the types.UnionType additions.

[skrawcz]

@mvanhorn all the test failures are related to this change. can you run the unit tests locally please?

ERROR tests/core/test_action.py - AttributeError: module 'types' has no attribute 'UnionType'
ERROR tests/core/test_application.py - AttributeError: module 'types' has no attribute 'UnionType'
ERROR tests/core/test_graph.py - AttributeError: module 'types' has no attribute 'UnionType'
ERROR tests/core/test_graphviz_display.py - AttributeError: module 'types' has no attribute 'UnionType'
ERROR tests/core/test_implementations.py - AttributeError: module 'types' has no attribute 'UnionType'
ERROR tests/core/test_parallelism.py - AttributeError: module 'types' has no attribute 'UnionType'
ERROR tests/core/test_persistence.py - AttributeError: module 'types' has no attribute 'UnionType'
ERROR tests/core/test_state.py - AttributeError: module 'types' has no attribute 'UnionType'
ERROR tests/core/test_validation.py - AttributeError: module 'types' has no attribute 'UnionType'
ERROR tests/integration_tests/test_app.py - AttributeError: module 'types' has no attribute 'UnionType'
ERROR tests/integrations/serde/test_langchain.py - AttributeError: module 'types' has no attribute 'UnionType'
ERROR tests/integrations/serde/test_pandas.py - AttributeError: module 'types' has no attribute 'UnionType'
ERROR tests/integrations/serde/test_pickle.py - AttributeError: module 'types' has no attribute 'UnionType'
ERROR tests/integrations/serde/test_pydantic.py - AttributeError: module 'types' has no attribute 'UnionType'
ERROR tests/integrations/test_burr_hamilton.py - AttributeError: module 'types' has no attribute 'UnionType'
ERROR tests/integrations/test_burr_haystack.py - AttributeError: module 'types' has no attribute 'UnionType'
ERROR tests/integrations/test_burr_opentelemetry.py - AttributeError: module 'types' has no attribute 'UnionType'
ERROR tests/integrations/test_burr_pydantic.py - AttributeError: module 'types' has no attribute 'UnionType'
ERROR tests/integrations/test_burr_pydantic_future_annotations.py - AttributeError: module 'types' has no attribute 'UnionType'
ERROR tests/integrations/test_burr_ray.py - AttributeError: module 'types' has no attribute 'UnionType'
ERROR tests/integrations/test_opentelemetry.py - AttributeError: module 'types' has no attribute 'UnionType'
ERROR tests/test_end_to_end.py - AttributeError: module 'types' has no attribute 'UnionType'
ERROR tests/tracking/test_common_models.py - AttributeError: module 'types' has no attribute 'UnionType'
ERROR tests/tracking/test_local_tracking_client.py - AttributeError: module 'types' has no attribute 'UnionType'

Otherwise, can you create an example that has:

stream_type=MyModel1 | MyModel2 # works

and that pyright doesn't complain about?

[mvanhorn]

@skrawcz good catch - that's a Python 3.9 import-time failure. types.UnionType was added in 3.10, and burr supports >=3.9 per pyproject.toml, so any 3.9 run hit AttributeError at test collection time before any test could run. Pyright was happy because I was running it on 3.10+, which is why I missed it.

Fixed in 6776e86: introduced a _UnionType compatibility alias in burr/core/action.py (types.UnionType on 3.10+, a placeholder sentinel class on 3.9) and replaced every types.UnionType reference in both burr/core/action.py and burr/integrations/pydantic.py. PEP 604 syntax doesn't exist on 3.9 so no real union can reach those call sites anyway - the sentinel path is just there to keep the annotations well-formed at import time.

Ran the full tests/core/{test_action,test_application,test_graph}.py suites in a clean venv: 250 passed.

[andreahlert]

tks @mvanhorn

LGTM after adding at least one test that exercises stream_type=Model1 | Model2 in Python 3.10+.

Optional but appreciated: fix the return-type annotation in _validate_and_extract_signature_types_streaming and consider renaming _UnionType to a public name.

[mvanhorn]

Done in 5c81fc8:

• Added test_streaming_pydantic_action_union_stream_type exercising stream_type=Model1 | Model2 (PEP 604 syntax), with a sys.version_info >= (3, 10) skip guard so 3.9 still passes.
• Widened the return annotation on _validate_and_extract_signature_types_streaming to Union[Type[dict], Type[pydantic.BaseModel], _UnionType] to match the runtime values returned in the union case.

Verified: pytest tests/integrations/test_burr_pydantic.py, 34 passed (Python 3.12).

Skipped the _UnionType to public-name rename for now. It's imported in burr/core/action.py (declaration, type alias, and the streaming_action annotation) and re-exported in burr/integrations/pydantic.py, so the rename has wider blast radius than this PR. If you'd like, I can do that as a follow-up once this lands so the rename PR stays focused.