[FEATURE] Option to infer response_model

Is your feature request related to a problem? Please describe. Primarily for the sake of the generated OpenAPI spec (but also for security/validation reasons), I currently specify a response_model for nearly all of my endpoints, but for the vast majority of my endpoints, the response_model is the same as the type that the endpoint returns.

This means that I am frequently duplicating the response_model in the return annotation.

In order to reduce duplication/room for copying/refactoring mistakes, it would be great if it were possible to (optionally) infer the response_model from the endpoint signature.

It should still be possible to override the value, and it should be ignored if a raw Response class is returned, I would just want the default to be the annotated return type of the endpoint. Note: this implementation would also make it much easier to catch anywhere the response_model differed from the returned value’s type while scanning through code.

To be clear, I don’t think this needs to be the default behavior, I just would like the option to enable it.

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I am currently using the following subclass of APIRouter in my code to enable this:

from typing import TYPE_CHECKING, Any, Callable, get_type_hints

from fastapi import APIRouter


class InferringRouter(APIRouter):
    if not TYPE_CHECKING:

        def add_api_route(self, path: str, endpoint: Callable[..., Any], **kwargs: Any) -> None:
            if kwargs.get("response_model") is None:
                kwargs["response_model"] = get_type_hints(endpoint).get("return")
            return super().add_api_route(path, endpoint, **kwargs)

    else:  # pragma: no cover
        pass

I think it would be nice if this capability was upstreamed though, both to ensure it is maintained, but also so others can benefit.

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Describe the solution you’d like

Rather than using the above implementation (which has some problems – for example, it isn’t possible to specify None as the response_model with an annotated return type), I think it would be better if there were a boolean-valued keyword argument to the APIRouter initializer that, if enabled, would cause the default behavior to be to have the annotated return type used as the response_model by default.

[dmontagu]

For anyone following this thread, I’ve released fastapi-utils which includes the InferringRouter class above.

You can read the docs here: https://fastapi-utils.davidmontague.xyz/user-guide/inferring-router/

[acnebs]

I think the API introduced by @dmontagu in fastapi_utils makes sense to integrate with FastAPI proper, with the addition of a response_model=None to override the return annotation as @tiangolo suggests.

Seems fairly trivial to handle this using a sentinel value.