Mounting sub-applications under APIRouter

[Kludex]

Discussed in #8682

Example Code

from fastapi import FastAPI, APIRouter
from starlette.testclient import TestClient

app = FastAPI()
api_router = APIRouter(prefix="/api")


@api_router.get("/app")
def read_main():
    return {"message": "Hello World from main app"}


app.include_router(api_router)

subapi = FastAPI()


@subapi.get("/sub")
def read_sub():
    return {"message": "Hello World from sub API"}


api_router.mount("/subapi", subapi)

client = TestClient(app)

assert client.get('/api/app').status_code == 200

# this next assert fails
assert client.get('/api/subapi/sub').status_code == 200

Description

Is it possible to mount a sub-application under an APIRouter? APIRouter itself has a mount function and accepts similar arguments to mounting a sub-application on a FastAPI instance, but I can't get the routing to actually work (nor can i get the openapi docs or spec to come back from that I would assume are the correct URLs.

The docs for sub applications note that the sub-application will have it's root_path correctly set, and I've tried a few combinations of manually setting the root_path on the subapi instance, but to no avail.

Related

• Mounting StaticFiles with an APIRouter doesn't work #9070
• 🐛 Fix mount subapp with APIRouter #5373

[Kludex]

Here we should either raise on APIRouter.mount or make it work.

[achaayb]

I dont see a reason of binding two FastAPI instances, perhaps in your usecase you're trying to merge two applications, in this case you should have a service that extracts endpoints and mounts them into an APIRouter

[baskervilski]

@Kludex , have you tried actually running the app and making a request to the endpoint of the subapp?

I have recently also struggled with a very similar setup, where I wanted to mount a v1 API and v2 API after a certain prefix, where I figured I should make a router to "hold" the /myapp/api/ prefix, then just mount to /v1 and /v2.

Long story short, I couldn't make it work that way, so I just mounted v1 app to myapp/api/v1 and v2 to myapp/api/v2 directly to the "root" app, no routers between.

EDIT: Also, I'm far from an expert, but I have a feeling that the TestClient class doesn't really "collaborate" with mounts... That's why I asked if you have tried to actually run the app and reach the endpoints.

[LorhanSohaky]

Maybe related #4657

[darkb0ts]

the issue is likely related to the way you are defining the routes within the sub-application (subapi). The routing paths within subapi should not include the prefix that you intend to use when mounting it under api_router.

[LorhanSohaky]

Even without including the prefix the problem occurs, as I reported on the issue #4656 making the doc link to be wrong

[mdczaplicki]

Any update on this? It's needed.

[mdczaplicki]

https://github.com/tiangolo/fastapi/blob/master/fastapi/routing.py#L812
This if-elsology doesn't provide a scenario for Mount.