Additional Status Codes

By default, FastAPI will return the responses using a JSONResponse, putting the content you return from your path operation inside of that JSONResponse.

It will use the default status code or the one you set in your path operation.

If you want to return additional status codes apart from the main one, you can do that by returning a Response directly, like a JSONResponse, and set the additional status code directly.

For example, let's say that you want to have a path operation that allows updating items, and returns HTTP status codes of 200 "OK" when successful.

But you also want it to accept new items. And when the items didn't exist before, it creates them, and returns an HTTP status code of 201 "Created".

To achieve that, import JSONResponse, and return your content there directly, setting the status_code that you want:

from typing import Annotated

from fastapi import Body, FastAPI, status
from fastapi.responses import JSONResponse

app = FastAPI()

items = {"foo": {"name": "Fighters", "size": 6}, "bar": {"name": "Tenders", "size": 3}}


@app.put("/items/{item_id}")
async def upsert_item(
    item_id: str,
    name: Annotated[str | None, Body()] = None,
    size: Annotated[int | None, Body()] = None,
):
    if item_id in items:
        item = items[item_id]
        item["name"] = name
        item["size"] = size
        return item
    else:
        item = {"name": name, "size": size}
        items[item_id] = item
        return JSONResponse(status_code=status.HTTP_201_CREATED, content=item)

If you return additional status codes and responses directly, they won't be included in the OpenAPI schema (the API docs), because FastAPI doesn't have a way to know beforehand what you are going to return.

But you can document that in your code, using: Additional Responses.