response_model does not invalidate None

[hukkin]

Example

from fastapi import FastAPI

from pydantic import BaseModel


app = FastAPI()


class RespModel(BaseModel):
    example_field: str


@app.get("/", response_model=RespModel)
def get_resp():
    return None

Description

• Open the browser and call the endpoint /.
• It returns a JSON with null, with HTTP code 200 OK.
• But I expected it to return HTTP 500, since I don't see how None is a valid RespModel .

Environment

• OS: Linux
• FastAPI Version: 0.63.0
• Python version: 3.8.6

Additional context

I'm not sure if this is a bug, or expected behavior. I tried to look for expected behavior in the docs but didn't find.

If this is expected, is there perhaps a way in the API to make FastAPI invalidate None when response_model is set?

[Kludex]

Looking at the code, it looks like it's a bug. Good job!

Let me explain why I think you're right and how this is happening:

On the APIRoute we have this line:

            self.response_field = create_response_field(
                name=response_name, type_=self.response_model
            )

Which is the line that determines the response behavior. The problem is not apparent just seeing this, so we need to follow the create_response_field() function in here:

def create_response_field(
    name: str,
    type_: Type[Any],
    class_validators: Optional[Dict[str, Validator]] = None,
    default: Optional[Any] = None,
    required: Union[bool, UndefinedType] = False,
    model_config: Type[BaseConfig] = BaseConfig,
    field_info: Optional[FieldInfo] = None,
    alias: Optional[str] = None,
) -> ModelField:
    ...

This is enough for us. Do you see the required field? It's the parameter that is causing what you experienced.

Just to be sure, I've changed the default value to True, and then I was able to get the 500 that you wanted.

Maybe adding a response_required field on the APIRoute can be a solution.

[hukkin]

Thanks for the explanation @Kludex. I took a stab at this here #2725

[wbadart]

@Kludex if I may ask a tangentially related question, why does FastAPI.post use a response_model parameter at all, rather than inspecting the return type annotation?

[Kludex]

I'm not going to be able to provide the link now and I'll probably forget later, but if you go to the docs and check "/response-model" path, there's a note about this. You should search for "type hint" on that page if I remember correctly.

[chbndrhnns]

why does FastAPI.post use a response_model parameter at all, rather than inspecting the return type annotation?

This is a response I once got to that question: #653 (comment)

[wbadart]

Gotcha. Thanks folks!

[tiangolo]

Indeed, this is a bug! Thanks @hukkin for the report, example code, and fix in the PR. 🚀

And thanks @Kludex for all the help and debugging! 🤓 🙇

The fix will be available in FastAPI 0.80.0, released in some hours. 🎉

[orenwang]

Okay, my server has complained a ton of errors since my yesterday's release. I suppose that's the price I'll have to pay for not pinning dependencies' versions...

[JarroVGIT]

What kind of errors? I haven’t updated yet, will try in the next few days

[Kludex]

If I'm not mistaken... I think the problem he mentions is that changing this behavior was a breaking change.

[orenwang]

@JarroVGIT Something like this:

pydantic.error_wrappers.ValidationError: 1 validation error for MyModel
  response
  none is not an allowed value (type=type_error.none.not_allowed)

But if you've never sent None as HTTP response, it shouldn't be a problem. If it is the case, the fix is easy: put Optional with whatever your response_model is.

[AlonMorgen]

Hi, I have some error regarding this change triggered by ddtrace (https://github.com/DataDog/dd-trace-py) library wrapping my fastapi application, but I think its not really, related to Datadog:

Upon getting the error after returning None instead of my response_model class (which in my case was "bool"), the datadog library gets the exception object generated by fastapi with type pydantic.error_wrappers.ValidationError which makes sense, because my route returned "None" where it should return "bool". But, then when it tries to stringify the object using the built in str function, It gets an exception regarding the self.model field of the ValidationError object:
AttributeError("type object 'bool' has no attribute 'pydantic_model'"), here:

def errors(self) -> List['ErrorDict']:
        if self._error_cache is None:
            try:
                config = self.model.__config__  # type: ignore
            except AttributeError:
                config = self.model.__pydantic_model__.__config__  # type: ignore
            self._error_cache = list(flatten_errors(self.raw_errors, config))
        return self._error_cache

apparently, the "model" field is just a "class bool" type because of the original response_model I put in my endpoint, and it doesn't have config nor pydantic_model fields, hence the exception.

any clue about that?

[tiangolo]

Thanks for the conversation all! ☕

@AlonMorgen if you're still having problems, please create a new GitHub Discussion following the template, with a replication example, etc. That way we would be able to help you.