fix(py): HttpErrorWireFormat JSON serialization in FastAPI/Flask

[huangjeff5]

When validation errors or flow exceptions occur, FastAPI and Flask handlers called json.dumps(get_callable_json(ex)) directly. get_callable_json() returns HttpErrorWireFormat (Pydantic BaseModel), which json.dumps cannot serialize — TypeError was raised and clients received 500 instead of 422/400.

Fix is to make sure get_callable_json returns a dict that can be json.dump() immediately.

Also enabled the case where you want to expose a flow directly as an HTTP function.

@app.post('/chat', response_model=None)
@genkit_fastapi_handler(ai)
@ai.flow()
async def chat(prompt: str) -> str:
    response = await ai.generate(prompt=prompt)
    return response.text

Previously you had to write:

@app.post('/chat', response_model=None)
@genkit_fastapi_handler(ai)
async def chat():
    return my_flow

@ai.flow()
async def chat(prompt: str) -> str:
    response = await ai.generate(prompt=prompt)
    return response.text

[gemini-code-assist]

Summary of Changes

Hello, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request resolves critical JSON serialization issues in FastAPI and Flask error handling, ensuring that API clients receive properly formatted error responses instead of internal server errors. It also enhances the robustness of stream processing by correctly propagating cancellation and exceptions, preventing unexpected crashes. These changes improve the reliability and user experience of applications built with Genkit's Python framework.

Highlights

• FastAPI/Flask Error Serialization Fix: Addressed an issue where HttpErrorWireFormat (a Pydantic BaseModel) was not correctly serialized to JSON in FastAPI and Flask error handlers, leading to TypeError and 500 responses instead of expected 4xx/5xx error codes with valid JSON.
• Stream Cancellation Robustness: Improved the send_chunk function in _action.py to gracefully handle stream cancellations and exceptions, ensuring they are propagated to the result_future without crashing the callback.
• Comprehensive Testing: Introduced new unit tests for FastAPI error handling to validate that both 400 (missing data) and 500 (flow exception) responses return valid JSON, and added a test for stream cancellation behavior.

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[gemini-code-assist]

Code Review

This pull request addresses a TypeError that occurred when serializing HttpErrorWireFormat Pydantic models to JSON in FastAPI and Flask error handlers. The fix correctly converts the Pydantic model to a dictionary before JSON serialization. The PR also includes a valuable improvement to streaming actions by properly handling cancellations and exceptions in callbacks, preventing potential crashes. New tests are added for both fixes. My review includes a couple of suggestions to make the new test assertions more specific and robust.

[huangjeff5]

/gemini review

[gemini-code-assist]

Code Review

This pull request correctly addresses a JSON serialization issue with HttpErrorWireFormat in FastAPI and Flask handlers. The core fix, which involves changing get_callable_json to return a dictionary, is sound and effectively prevents the TypeError. The changes are well-supported by updated unit tests and new integration tests for FastAPI that confirm the fix. I've included a couple of suggestions to enhance code consistency and test maintainability.