I want my FastAPI application to return JSON:API responses. It would be really cumbersome to craft each response manually so I’ve looked into creating a custom response class, one based on JSONResponse, since the responses have to be in JSON format.

They have an example here:

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

So, subclass your response class (in my case JSONResponse), subclass it, implement the render method.

So far so good:

class JSONAPIResponse(JSONResponse):

    def render(self, content: Any) -> Any:

        # Null object.
        if content is None:
    [...]

However, the problem comes at the next step.[1]

If I add a content_object_type to JSONAPIResponse, I put it in the __init__ method and everything, FastAPI still complains about it.

class JSONAPIResponse(JSONResponse):

    content_object_type: str

    def __init__(self, content: Any, content_object_type: str):
        super().__init__(content=content)

        self.content_object_type = content_object_type

    def render(self, content: Any) -> Any:

        self.content_object_type

    [... Elsewhere in the code base, another method ...]

    return JSONAPIResponse(content=jsonable_encoder(content_object), content_object_type="bla")

Boom ???? this is the FastAPI error message below

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<code>def render(self, content: Any) -> Any:

self.content_object_type

</code>

<code>def render(self, content: Any) -> Any: self.content_object_type </code>

def render(self, content: Any) -> Any:

  self.content_object_type

E AttributeError: ‘JSONAPIResponse’ object has no attribute ‘content_object_type’

So I’ve declared the attribute, I have it in the constructor that’s used… what’s wrong?