Custom Response - HTML, Stream, File, others¶

By default, FastAPI will return JSON responses.

You can override it by returning a Response directly as seen in Return a Response directly.

But if you return a Response directly (or any subclass, like JSONResponse), the data won't be automatically converted (even if you declare a response_model), and the documentation won't be automatically generated (for example, including the specific "media type", in the HTTP header Content-Type as part of the generated OpenAPI).

But you can also declare the Response that you want to be used (e.g. any Response subclass), in the path operation decorator using the response_class parameter.

The contents that you return from your path operation function will be put inside of that Response.

Note

If you use a response class with no media type, FastAPI will expect your response to have no content, so it will not document the response format in its generated OpenAPI docs.

JSON Responses¶

By default FastAPI returns JSON responses.

If you declare a Response Model FastAPI will use it to serialize the data to JSON, using Pydantic.

If you don't declare a response model, FastAPI will use the jsonable_encoder explained in JSON Compatible Encoder and put it in a JSONResponse.

If you declare a response_class with a JSON media type (application/json), like is the case with the JSONResponse, the data you return will be automatically converted (and filtered) with any Pydantic response_model that you declared in the path operation decorator. But the data won't be serialized to JSON bytes with Pydantic, instead it will be converted with the jsonable_encoder and then passed to the JSONResponse class, which will serialize it to bytes using the standard JSON library in Python.

JSON Performance¶

In short, if you want the maximum performance, use a Response Model and don't declare a response_class in the path operation decorator.

Python 3.10+

# Code above omitted 👆

@app.post("/items/")
async def create_item(item: Item) -> Item:
    return item

# Code below omitted 👇

👀 Full file preview

Python 3.10+

from fastapi import FastAPI
from pydantic import BaseModel

app = FastAPI()


class Item(BaseModel):
    name: str
    description: str | None = None
    price: float
    tax: float | None = None
    tags: list[str] = []


@app.post("/items/")
async def create_item(item: Item) -> Item:
    return item


@app.get("/items/")
async def read_items() -> list[Item]:
    return [
        Item(name="Portal Gun", price=42.0),
        Item(name="Plumbus", price=32.0),
    ]

HTML Response¶

To return a response with HTML directly from FastAPI, use HTMLResponse.

Import HTMLResponse.
Pass HTMLResponse as the parameter response_class of your path operation decorator.

Python 3.10+

from fastapi import FastAPI
from fastapi.responses import HTMLResponse

app = FastAPI()


@app.get("/items/", response_class=HTMLResponse)
async def read_items():
    return """
    <html>
        <head>
            <title>Some HTML in here</title>
        </head>
        <body>
            <h1>Look ma! HTML!</h1>
        </body>
    </html>
    """

Info

The parameter response_class will also be used to define the "media type" of the response.

In this case, the HTTP header Content-Type will be set to text/html.

And it will be documented as such in OpenAPI.

Return a `Response`¶

As seen in Return a Response directly, you can also override the response directly in your path operation, by returning it.

The same example from above, returning an HTMLResponse, could look like:

Python 3.10+

from fastapi import FastAPI
from fastapi.responses import HTMLResponse

app = FastAPI()


@app.get("/items/")
async def read_items():
    html_content = """
    <html>
        <head>
            <title>Some HTML in here</title>
        </head>
        <body>
            <h1>Look ma! HTML!</h1>
        </body>
    </html>
    """
    return HTMLResponse(content=html_content, status_code=200)

Warning

A Response returned directly by your path operation function won't be documented in OpenAPI (for example, the Content-Type won't be documented) and won't be visible in the automatic interactive docs.

Info

Of course, the actual Content-Type header, status code, etc, will come from the Response object you returned.

Document in OpenAPI and override `Response`¶

If you want to override the response from inside of the function but at the same time document the "media type" in OpenAPI, you can use the response_class parameter AND return a Response object.

The response_class will then be used only to document the OpenAPI path operation, but your Response will be used as is.

Return an `HTMLResponse` directly¶

For example, it could be something like:

Python 3.10+

from fastapi import FastAPI
from fastapi.responses import HTMLResponse

app = FastAPI()


def generate_html_response():
    html_content = """
    <html>
        <head>
            <title>Some HTML in here</title>
        </head>
        <body>
            <h1>Look ma! HTML!</h1>
        </body>
    </html>
    """
    return HTMLResponse(content=html_content, status_code=200)


@app.get("/items/", response_class=HTMLResponse)
async def read_items():
    return generate_html_response()

In this example, the function generate_html_response() already generates and returns a Response instead of returning the HTML in a str.

By returning the result of calling generate_html_response(), you are already returning a Response that will override the default FastAPI behavior.

But as you passed the HTMLResponse in the response_class too, FastAPI will know how to document it in OpenAPI and the interactive docs as HTML with text/html:

Available responses¶

Here are some of the available responses.

Keep in mind that you can use Response to return anything else, or even create a custom sub-class.

Technical Details

You could also use from starlette.responses import HTMLResponse.

FastAPI provides the same starlette.responses as fastapi.responses just as a convenience for you, the developer. But most of the available responses come directly from Starlette.

`Response`¶

The main Response class, all the other responses inherit from it.

You can return it directly.

It accepts the following parameters:

content - A str or bytes.
status_code - An int HTTP status code.
headers - A dict of strings.
media_type - A str giving the media type. E.g. "text/html".

FastAPI (actually Starlette) will automatically include a Content-Length header. It will also include a Content-Type header, based on the media_type and appending a charset for text types.

Python 3.10+

from fastapi import FastAPI, Response

app = FastAPI()


@app.get("/legacy/")
def get_legacy_data():
    data = """<?xml version="1.0"?>
    <shampoo>
    <Header>
        Apply shampoo here.
    </Header>
    <Body>
        You'll have to use soap here.
    </Body>
    </shampoo>
    """
    return Response(content=data, media_type="application/xml")

`HTMLResponse`¶

Takes some text or bytes and returns an HTML response, as you read above.

`PlainTextResponse`¶

Takes some text or bytes and returns a plain text response.

Python 3.10+

from fastapi import FastAPI
from fastapi.responses import PlainTextResponse

app = FastAPI()


@app.get("/", response_class=PlainTextResponse)
async def main():
    return "Hello World"

`JSONResponse`¶

Takes some data and returns an application/json encoded response.

This is the default response used in FastAPI, as you read above.

Technical Details

But if you declare a response model or return type, that will be used directly to serialize the data to JSON, and a response with the right media type for JSON will be returned directly, without using the JSONResponse class.

This is the ideal way to get the best performance.

`RedirectResponse`¶

Returns an HTTP redirect. Uses a 307 status code (Temporary Redirect) by default.

You can return a RedirectResponse directly:

Python 3.10+

from fastapi import FastAPI
from fastapi.responses import RedirectResponse

app = FastAPI()


@app.get("/typer")
async def redirect_typer():
    return RedirectResponse("https://typer.tiangolo.com")

Or you can use it in the response_class parameter:

Python 3.10+

from fastapi import FastAPI
from fastapi.responses import RedirectResponse

app = FastAPI()


@app.get("/fastapi", response_class=RedirectResponse)
async def redirect_fastapi():
    return "https://fastapi.tiangolo.com"

If you do that, then you can return the URL directly from your path operation function.

In this case, the status_code used will be the default one for the RedirectResponse, which is 307.

You can also use the status_code parameter combined with the response_class parameter:

Python 3.10+

from fastapi import FastAPI
from fastapi.responses import RedirectResponse

app = FastAPI()


@app.get("/pydantic", response_class=RedirectResponse, status_code=302)
async def redirect_pydantic():
    return "https://docs.pydantic.dev/"

`StreamingResponse`¶

Takes an async generator or a normal generator/iterator (a function with yield) and streams the response body.

Python 3.10+

import anyio
from fastapi import FastAPI
from fastapi.responses import StreamingResponse

app = FastAPI()


async def fake_video_streamer():
    for i in range(10):
        yield b"some fake video bytes"
        await anyio.sleep(0)


@app.get("/")
async def main():
    return StreamingResponse(fake_video_streamer())

Technical Details

An async task can only be cancelled when it reaches an await. If there is no await, the generator (function with yield) can not be cancelled properly and may keep running even after cancellation is requested.

Since this small example does not need any await statements, we add an await anyio.sleep(0) to give the event loop a chance to handle cancellation.

This would be even more important with large or infinite streams.

Tip

Instead of returning a StreamingResponse directly, you should probably follow the style in Stream Data, it's much more convenient and handles cancellation behind the scenes for you.

If you are streaming JSON Lines, follow the Stream JSON Lines tutorial.

`FileResponse`¶

Asynchronously streams a file as the response.

Takes a different set of arguments to instantiate than the other response types:

path - The file path to the file to stream.
headers - Any custom headers to include, as a dictionary.
media_type - A string giving the media type. If unset, the filename or path will be used to infer a media type.
filename - If set, this will be included in the response Content-Disposition.

File responses will include appropriate Content-Length, Last-Modified and ETag headers.

Python 3.10+

from fastapi import FastAPI
from fastapi.responses import FileResponse

some_file_path = "large-video-file.mp4"
app = FastAPI()


@app.get("/")
async def main():
    return FileResponse(some_file_path)

You can also use the response_class parameter:

Python 3.10+

from fastapi import FastAPI
from fastapi.responses import FileResponse

some_file_path = "large-video-file.mp4"
app = FastAPI()


@app.get("/", response_class=FileResponse)
async def main():
    return some_file_path

In this case, you can return the file path directly from your path operation function.

Custom response class¶

You can create your own custom response class, inheriting from Response and using it.

For example, let's say that you want to use orjson with some settings.

Let's say you want it to return indented and formatted JSON, so you want to use the orjson option orjson.OPT_INDENT_2.

You could create a CustomORJSONResponse. The main thing you have to do is create a Response.render(content) method that returns the content as bytes:

Python 3.10+

from typing import Any

import orjson
from fastapi import FastAPI, Response

app = FastAPI()


class CustomORJSONResponse(Response):
    media_type = "application/json"

    def render(self, content: Any) -> bytes:
        assert orjson is not None, "orjson must be installed"
        return orjson.dumps(content, option=orjson.OPT_INDENT_2)


@app.get("/", response_class=CustomORJSONResponse)
async def main():
    return {"message": "Hello World"}

Now instead of returning:

{"message": "Hello World"}

...this response will return:

{
  "message": "Hello World"
}

Of course, you will probably find much better ways to take advantage of this than formatting JSON. 😉

`orjson` or Response Model¶

If what you are looking for is performance, you are probably better off using a Response Model than an orjson response.

With a response model, FastAPI will use Pydantic to serialize the data to JSON, without using intermediate steps, like converting it with jsonable_encoder, which would happen in any other case.

And under the hood, Pydantic uses the same underlying Rust mechanisms as orjson to serialize to JSON, so you will already get the best performance with a response model.

Default response class¶

When creating a FastAPI class instance or an APIRouter you can specify which response class to use by default.

The parameter that defines this is default_response_class.

In the example below, FastAPI will use HTMLResponse by default, in all path operations, instead of JSON.

Python 3.10+

from fastapi import FastAPI
from fastapi.responses import HTMLResponse

app = FastAPI(default_response_class=HTMLResponse)


@app.get("/items/")
async def read_items():
    return "<h1>Items</h1><p>This is a list of items.</p>"

Tip

You can still override response_class in path operations as before.

Additional documentation¶

You can also declare the media type and many other details in OpenAPI using responses: Additional Responses in OpenAPI.

Custom Response - HTML, Stream, File, others¶

JSON Responses¶

JSON Performance¶

HTML Response¶

Return a Response¶

Document in OpenAPI and override Response¶

Return an HTMLResponse directly¶

Available responses¶

Response¶

HTMLResponse¶

PlainTextResponse¶

JSONResponse¶

RedirectResponse¶

StreamingResponse¶

FileResponse¶

Custom response class¶

orjson or Response Model¶

Default response class¶

Additional documentation¶

Return a `Response`¶

Document in OpenAPI and override `Response`¶

Return an `HTMLResponse` directly¶

`Response`¶

`HTMLResponse`¶

`PlainTextResponse`¶

`JSONResponse`¶

`RedirectResponse`¶

`StreamingResponse`¶

`FileResponse`¶

`orjson` or Response Model¶