Why should I return ORJSONResponse

[idan3]

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Description

In the documentation, the example is:

@app.get("/items/", response_class=ORJSONResponse)
async def read_items():
    return ORJSONResponse([{"item_id": "Foo"}])

I do not understand why I need to do ORJSONResponse([{"item_id": "Foo"}])? This works as well:

@app.get("/items/", response_class=ORJSONResponse)
async def read_items():
    return [{"item_id": "Foo"}]   # Or: return MyResponse("hello")

It also works with class objects and is more readable and still (If I understand correctly) orjson is the one doing the serialization.

Operating System

macOS

Operating System Details

No response

FastAPI Version

0.109.2

Pydantic Version

2.6.1

Python Version

Python 3.10.13

Additional Context

No response

[YuriiMotov]

It's for optimization purposes. This can be useful if your return data is big and you are sure that it's serializable with JSON.

As it's said in the documentation, by returning ORJSONResponse([{"item_id": "Foo"}]) you can

avoid the extra overhead that FastAPI would have by passing your return content through the jsonable_encoder before passing it to the response class.

https://fastapi.tiangolo.com/advanced/custom-response/#use-orjsonresponse

ORJSONResponse is inherited from the Response class. So, in this case you will save some execution time by not passing your return content through the jsonable_encoder. If you just return [{"item_id": "Foo"}], FastAPI will pass it through the jsonable_encoder and then validate.