Also raised on #26 (comment). See also #544.

Describe the bug

Write here a clear and concise description of what the bug is.

To Reproduce

Replace each part with your own scenario:

1. Create a file with:

from fastapi import FastAPI

app = FastAPI()

@app.get("/app")
def read_root():
    return {"Hello": "World"}

2. Launch it using uvicorn --root-path="bar" test_app:app
3. Open the browser and go to http://127.0.0.1:8000/docs.
4. From the documentation, call the GET /app route.
5. The doc page calls /app and succeeds.

Expected behavior

The above test should fail after having called /bar/app, since root_path is supposed to prefix all generated URLs in case the application is served behind a reverse-proxy, among ther things. FastAPI only acknowledges openapi_prefix for the API doc.

Environment

• OS: Windows
• FastAPI Version: 0.45.0
• Python version: 3.8.0

Additional context

A similar issue applies to sub-applications:

from fastapi import FastAPI

app = FastAPI()


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


#subapi = FastAPI(openapi_prefix="/subapi")
subapi = FastAPI()

@subapi.get("/sub")
def read_sub(request: Request):
    return {
        "root_path": request.scope['root_path'],
        "raw_path": request.scope['raw_path'],
        "path": request.scope['path'],
        "app_url_for": app.url_path_for("read_sub"),
        "subapp_url_for": subapi.url_path_for("read_sub"),
    }

app.mount("/subapi", subapi)

{
  "root_path":"bar/subapi",
  "raw_path":"/subapi/sub",
  "path":"/sub",
  "app_url_for":"/subapi/sub",
  "subapp_url_for":"/sub"
}

(url_for not being prefixed with root_path is fixed upstream by Kludex/starlette#699)

Unless openapi_prefix="/subapi" is passed when creating the subapplication, both http://127.0.0.1:8000/docs and http://127.0.0.1:8000/subapi/docs will point towards http://127.0.0.1:8000/openapi.json, which goes against the point of having isolated subapplications.

openapi_prefix should probably just be deprecated and assumed to match root_path if absent.