Multi-level router can't use empty path (eg. get.("") )

[jordantshaw]

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Example Code

# main.py

from fastapi import FastAPI, APIRouter
from .child import child_router

app = FastAPI()

router = APIRouter()
router.include_router(child_router, prefix='/foo')

app.include_router(router, prefix='/api/v1', include_in_schema=True)

# .child/__init__.py
from fastapi import APIRouter

from .grandchild import grandchild_router

child_router = APIRouter()
child_router.include_router(grandchild_router)

# ..grandchild/__init__.py
from fastapi import APIRouter

grandchild_router = APIRouter()

@grandchild_router.get('')
def test():
    return 'bar'

Description

I currently have something similar setup as the example code. main_router > child_router > grandchild_router. I am setting a prefix on the child router level (during the include_router call). When I set the prefix on the child level, I cannot use an empty path on the grandchild level router.

If I do, I got the following error:
Exception: Prefix and path cannot be both empty (path operation: test)

My file structure looks like this.

project
│   main.py (main_router)
└───child
│   │   __init__.py (child_router)
│   └───grandchild
│       │    __init__.py (grandchild_router)


### Operating System

macOS

### Operating System Details

_No response_

### FastAPI Version

0.75.0

### Python Version

Python 3.9.10

### Additional Context

_No response_

[gmanny]

Looks like routers are meant to be in a tree structure, child routers having a path inside.

This is related to the check on this line. As there is no lookup tree, and routers are really just combined into a big routing list I would say checking in the original route + prefix if that the router actually has an empty route first, should be easy and would relieve this situation. (Another option is checking if a route with the path exists in the APIRouter.add_api_route, but this is open to all kinds of trouble - i.e. what is there are two identical paths where only variable names are different.)

[mustyoshi]

I've also just run into this issue and had to settle for having a trailing slash on the child route.

[gmanny]

I've ended up with this function, which I apply to the route router after adding all the child routers:

from typing import TypeVar

from fastapi import FastAPI
from fastapi.routing import APIRoute, APIRouter

ParentT = TypeVar("ParentT", APIRouter, FastAPI)


def remove_trailing_slashes_from_routes(parent: ParentT) -> ParentT:
    "Removes trailing slashes from all routes in the given router"

    for route in parent.routes:
        if isinstance(route, APIRoute):
            route.path = route.path.rstrip("/")

    return parent