From 14d7d105d47436e9fabffe9d79703af98074239b Mon Sep 17 00:00:00 2001
From: majiayu000 <1835304752@qq.com>
Date: Tue, 30 Dec 2025 14:23:46 +0800
Subject: [PATCH] fix(pydantic): support union type hints for stream_type
 parameter

Updated type hints to accept Union types for stream_type parameter
in streaming_action.pydantic decorator. This allows users to specify
union types like Union[Model1, Model2] for stream_type.

Closes #607

Signed-off-by: majiayu000 <1835304752@qq.com>
---
 burr/core/action.py                      |  2 +-
 burr/integrations/pydantic.py            |  8 ++--
 tests/integrations/test_burr_pydantic.py | 51 +++++++++++++++++++++++-
 3 files changed, 54 insertions(+), 7 deletions(-)

diff --git a/burr/core/action.py b/burr/core/action.py
index 69a7c75b6..8b42c8817 100644
--- a/burr/core/action.py
+++ b/burr/core/action.py
@@ -1309,7 +1309,7 @@ def pydantic(
         writes: List[str],
         state_input_type: Type["BaseModel"],
         state_output_type: Type["BaseModel"],
-        stream_type: Union[Type["BaseModel"], Type[dict]],
+        stream_type: Union[Type["BaseModel"], Type[dict], type, "types.UnionType"],
         tags: Optional[List[str]] = None,
     ) -> Callable:
         """Creates a streaming action that uses pydantic models.
diff --git a/burr/integrations/pydantic.py b/burr/integrations/pydantic.py
index 5354537a2..bca543c40 100644
--- a/burr/integrations/pydantic.py
+++ b/burr/integrations/pydantic.py
@@ -264,7 +264,7 @@ async def async_action_function(state: State, **kwargs) -> State:
     return decorator
 
 
-PartialType = Union[Type[pydantic.BaseModel], Type[dict]]
+PartialType = Union[Type[pydantic.BaseModel], Type[dict], type, "types.UnionType"]
 
 PydanticStreamingActionFunctionSync = Callable[
     ..., Generator[Tuple[Union[pydantic.BaseModel, dict], Optional[pydantic.BaseModel]], None, None]
@@ -285,12 +285,10 @@ async def async_action_function(state: State, **kwargs) -> State:
 
 def _validate_and_extract_signature_types_streaming(
     fn: PydanticStreamingActionFunction,
-    stream_type: Optional[Union[Type[pydantic.BaseModel], Type[dict]]],
+    stream_type: Optional[PartialType],
     state_input_type: Optional[Type[pydantic.BaseModel]] = None,
     state_output_type: Optional[Type[pydantic.BaseModel]] = None,
-) -> Tuple[
-    Type[pydantic.BaseModel], Type[pydantic.BaseModel], Union[Type[dict], Type[pydantic.BaseModel]]
-]:
+) -> Tuple[Type[pydantic.BaseModel], Type[pydantic.BaseModel], PartialType]:
     if stream_type is None:
         # TODO -- derive from the signature
         raise ValueError(f"stream_type is required for function: {fn.__qualname__}")
diff --git a/tests/integrations/test_burr_pydantic.py b/tests/integrations/test_burr_pydantic.py
index 6c25520f6..16dc1a3f6 100644
--- a/tests/integrations/test_burr_pydantic.py
+++ b/tests/integrations/test_burr_pydantic.py
@@ -16,7 +16,7 @@
 # under the License.
 
 import asyncio
-from typing import AsyncGenerator, Generator, List, Optional, Tuple
+from typing import AsyncGenerator, Generator, List, Optional, Tuple, Union
 
 import pydantic
 import pytest
@@ -758,3 +758,52 @@ async def final_result_streamed(
     assert state.data.count == 20
     assert isinstance(result, IntermediateModel)
     assert result.result == 20
+
+
+class StreamTypeModel1(BaseModel):
+    value: int
+
+
+class StreamTypeModel2(BaseModel):
+    message: str
+
+
+def test_streaming_pydantic_action_union_stream_type():
+    """Test that stream_type accepts union types like Union[Model1, Model2]."""
+    union_type = Union[StreamTypeModel1, StreamTypeModel2]
+
+    @pydantic_streaming_action(
+        reads=["count", "times_called"],
+        writes=["count", "times_called"],
+        stream_type=union_type,
+        state_input_type=AppStateModel,
+        state_output_type=AppStateModel,
+    )
+    def act(
+        state: AppStateModel, total_count: int
+    ) -> Generator[
+        Tuple[Union[StreamTypeModel1, StreamTypeModel2], Optional[AppStateModel]], None, None
+    ]:
+        for i in range(total_count):
+            if i % 2 == 0:
+                yield StreamTypeModel1(value=i), None
+            else:
+                yield StreamTypeModel2(message=f"step_{i}"), None
+            state.count = i
+        state.times_called += 1
+        yield StreamTypeModel1(value=state.count), state
+
+    assert hasattr(act, "bind")
+    action_function = getattr(act, FunctionBasedAction.ACTION_FUNCTION, None)
+    assert action_function is not None
+    gen = action_function.fn(
+        State(dict(count=0, times_called=0), typing_system=PydanticTypingSystem(AppStateModel)),
+        total_count=4,
+    )
+    result = list(gen)
+    assert len(result) == 5
+    assert isinstance(result[0][0], StreamTypeModel1)
+    assert isinstance(result[1][0], StreamTypeModel2)
+    assert isinstance(result[2][0], StreamTypeModel1)
+    assert isinstance(result[3][0], StreamTypeModel2)
+    assert isinstance(result[-1][1], State)