During the use of FastAPI, I noticed a problem with sub-routes, My project structure is as follows:

┌─example
│ ├─main.py
│ └─sub_dir
│   ├─__init__.py
│   └─static
│     └─index.html

The code is as follows:

# example/main.py
from fastapi import FastAPI

from sub_dir import sub_dir

app = FastAPI()
app.include_router(sub_dir, prefix="/app")

if __name__ == "__main__":
    import uvicorn

    uvicorn.run(app, host="127.0.0.1", port=8000)

# example/sub_dir/__init__.py
from pathlib import Path as libPath

from fastapi import APIRouter
from fastapi.staticfiles import StaticFiles

skip = APIRouter()
skip.mount("/static", StaticFiles(directory=libPath(__file__).resolve().parent / "static"), name="skip")

__all__ = ["skip"]

After running main.py and accessing http://127.0.0.1:8000/app/sub_dir/index.html, one should have seen the content of example/sub_dir/static/index.html. However, it did not display as expected, Log entry as follows:

INFO:     Started server process [60576]
INFO:     Waiting for application startup.
INFO:     Application startup complete.
INFO:     Uvicorn running on http://127.0.0.1:8000 (Press CTRL+C to quit)
INFO:     127.0.0.1:59030 - "GET /app/sub_dir/index.html HTTP/1.1" 404 Not Found

By examining the source code, it was discovered that APIRouter.include_router failed to properly handle routing.Mount.

After the repair, we ran main.py, and we could see that the content in example/sub_dir/static/index.html could be displayed normally. The log is as follows:

INFO:     Started server process [78196]
INFO:     Waiting for application startup.
INFO:     Application startup complete.
INFO:     Uvicorn running on http://127.0.0.1:8000 (Press CTRL+C to quit)
INFO:     127.0.0.1:58825 - "GET /app/sub_dir/index.html HTTP/1.1" 200 OK

This will be extremely useful in complex routing scenarios!!!