fastapi/references/responses.md

Responses

Return Type or Response Model

When possible, include a return type. It will be used to validate, filter, document, and serialize the response.

from fastapi import FastAPI
from pydantic import BaseModel

app = FastAPI()


class Item(BaseModel):
    name: str
    description: str | None = None


@app.get("/items/me")
async def get_item() -> Item:
    return Item(name="Plumbus", description="All-purpose home device")

Return types or response models filter data to avoid exposing sensitive information. They also let Pydantic serialize data on the Rust side for performance.

The return type doesn't have to be a Pydantic model. It can be a different type, like a list of integers, a dict, etc.

When to use response_model

If the return type is not the same as the type that you want to use to validate, filter, or serialize, use the response_model parameter on the decorator.

from typing import Any

from fastapi import FastAPI
from pydantic import BaseModel

app = FastAPI()


class Item(BaseModel):
    name: str
    description: str | None = None


@app.get("/items/me", response_model=Item)
async def get_item() -> Any:
    return {"name": "Foo", "description": "A very nice Item"}

This is particularly useful when filtering data to expose only the public fields and avoid exposing sensitive information.

from typing import Any

from fastapi import FastAPI
from pydantic import BaseModel

app = FastAPI()


class InternalItem(BaseModel):
    name: str
    description: str | None = None
    secret_key: str


class Item(BaseModel):
    name: str
    description: str | None = None


@app.get("/items/me", response_model=Item)
async def get_item() -> Any:
    item = InternalItem(
        name="Foo", description="A very nice Item", secret_key="supersecret"
    )
    return item