How to load a different file than index.html in FastAPI root path while using StaticFiles?

Here is a simple static FastAPI app. With this setup even though the root path is expected to return a FileResponse of custom.html, the app still returns index.html. How can I get the root path work and render custom.html?

from fastapi import FastAPI
from fastapi.staticfiles import StaticFiles
from fastapi.responses import FileResponse

app = FastAPI()


app.mount(
    "/",
    StaticFiles(directory="static", html=True),
    name="static",
)

@app.get("/")
async def index() -> FileResponse:
    return FileResponse("custom.html", media_type="html")

As per Starlette documentation:

StaticFiles

Signature: StaticFiles(directory=None, packages=None, html=False, check_dir=True, follow_symlink=False)

• html - Run in HTML mode. Automatically loads index.html for directories if such file exists.

In addtion, as shown from the code snippet you provided, you have mounted StaticFiles to the root directory (i.e., /), that is:

app.mount('/', StaticFiles(directory='static', html=True), name='static')

instead of, for example, /static (or some other path name), as shown below:

from fastapi import FastAPI
from fastapi.staticfiles import StaticFiles

app = FastAPI()

app.mount('/static', StaticFiles(directory='static', html=True), name='static')

As per FastAPI documentation:

"Mounting" means adding a complete "independent" application in a specific path, that then takes care of handling all the sub-paths.

Hence, in your example, any path that starts with / will be handled by that StaticFiles application, and due to specifying html=True in the arguments, index.html will be automatically loaded; regardless of creating a separate endpoint pointing to the root path / and trying to return some other file, as demonstrated in the example given in your question.

Order Matters

If, for example, you moved app.mount("/",StaticFiles(... line after defining your @app.get("/") endpoint, you would see that order matters and index.html would not automatically be loaded anymore, as endpoints are evaluated in order. Note that, in your case, you might get an Internal Server Error, as your @app.get("/") endpoint would be called and attempt to find custom.html, but if this file is not located under the root / directory, but rather under /static directory (as shown from your code), you would then get a File does not exist error, and hence, you should instead return FileResponse('static/custom.html').

Even if you removed html=True, but keep StaticFiles mounted to the root directory and defined before your / endpoint, you would get a {"detail":"Not Found"} error response when attempting to access http://localhost:8000/. This is because the / route would still be handled by the StaticFiles application (as mentioned earlier), and you should thus need to specify the file that you would like to access (when html=True is not used), e.g., http://localhost:8000/index.html. Even if you defined other endpoints in your code (e.g., /register, /login, /hello), as long as StaticFiles is mounted to the root directory (i.e., /) and defined in your code before all other endpoints, for instance:

app.mount('/', StaticFiles(directory='static'), name='static')

@app.post('/register')
async def register():
    pass

@app.post('/login')
async def login():
    pass

@app.get('/hello')
async def hello():
    pass

every request to those routes would again be handled by the StaticFiles application, and hence, would lead to an error response, such as {"detail":"Not Found"} (if you send a GET request, such as when you type a URL in the address bar of the web browser and then hit the Enter key, and the given path does not match a file name in the static web directory), or {detail": "Method Not Allowed"} (if you issue a POST request through Swagger UI or some other client platform/application). As described in Starlette's documentation on StaticFiles (see StaticFiles class implementation as well):

Static files will respond with 404 Not found or 405 Method not allowed responses for requests which do not match. In HTML mode, if 404.html file exists, it will be shown as 404 response.

Hence, you should either mount the StaticFiles instance to a different/unique path, such as /static (i.e., app.mount('/static', ..., as shown at the top of this answer), or, if you still want to mount the StaticFiles instance to / path, define StaticFiles after declaring all your API endpoints, for example:

from fastapi import FastAPI
from fastapi.staticfiles import StaticFiles
from fastapi.responses import FileResponse

app = FastAPI()


@app.post('/register')
async def register():
    pass


@app.post('/login')
async def login():
    pass


@app.get('/hello')
async def hello():
    pass


@app.get('/')
async def index():
    return FileResponse('static/custom.html')
 
 
app.mount('/',StaticFiles(directory='static', html=True), name='static')

Note

Every time a webpage is loaded, the web browser caches most content on the page, in order to shorten laod times (the next time that user loads the page). Thus, if you tried out the example provided earlier, i.e., where the StaticFiles application is defined before every API endpoint, and then, using the same browser session, you tried out the example above with the StaticFiles application defined after all API endpoints, but the browser still displays the content of static/index.html file instead of static/custom.html—when accessing http://localhost:8000/ in your browser—this is due to the browser loading the webpage from the cache. To overcome this, you could either clear your browser's cache, or open the webpage in an Incognito window (and close it when you are done with it), or simply press Ctrl+F5, instead of just F5, in your browser (using either an Incognito or regular window), which would force the browser to retrieve the webpage from the server instead of loading it from the cache.

You may also find this answer helpful, regarding the order of endpoints in FastAPI.

The html=True option

Setting the html argument of StaticFiles instance to True (i.e., html=True) simply provides an easy way to serve a directory of web content with just one line of code. If you only need to serve static files, such as package docs directory, then this is the way to go. If, however, you need to serve different HTML files that will get dynamically updated, as well as you wish to create additional routes/endpoints, you should better have a look at Templates (not FileResponse), as well as mount your StaticFiles instance to a different path (e.g., /static), rather than root path (and without using html=True).