[BUG] Path parameters not set in starlette Request object

Describe the bug

I am trying to access the path_params for a request from the middleware using request.path_params but it shows an empty {}.

However, it works if I access the Request object from within the endpoint code.

To Reproduce

Steps to reproduce the behavior with a minimum self-contained file.

from fastapi import FastAPI

app = FastAPI()
from starlette.requests import Request

@app.get('/fib-feed/{hello}/hellos')
async def test(path: str, query1: str, query2: str, request: Request):
    print(request.path_params)     # Is correctly populated


@app.middleware("http")
async def middleware(request: Request, call_next):
    print(request.path_params)     # Empty here

Expected behavior

The path_params should be accessible in middleware too.

Environment

• OS: macOS

• FastAPI Version : 0.20.0 [e.g. 0.3.0], get it with:

• Python version, get it with: 3.6.7

[nsidnev]

@shalseban Hi! I’m not sure if this should be a FastAPI issue. FastAPI does nothing with the middleware system provided by Starlette, so I think that this issue should be opened in their repository. The main problem here is that path_params in Request object are filled when the route is checked for matching with the request, and this happens after the middleware called call_next callable. This way you can get path_params in middleware, but only after your route has returned the response.

As a workaround you can manualy iterate over your application routes and get path_params, as the Router class from Starlette does when matching request:

from typing import Callable

from fastapi import FastAPI
from starlette.requests import Request
from starlette.responses import Response
from starlette.routing import Match

app = FastAPI()


@app.middleware("http")
async def my_middleware(request: Request, call_next: Callable) -> Response:
    routes = request.app.router.routes
    for route in routes:
        match, scope = route.matches(request)
        if match == Match.FULL:
            print(scope["path_params"])

    return await call_next(request)


@app.get("/{param}")
def my_route(param: str) -> dict:
    return {"param": param}

[tiangolo]

Thanks for the help here @nsidnev! 🙇

And thanks for coming back to close the issue @shalseban. 🍪

This is indeed not really related to FastAPI. And I think it wouldn’t be easily doable in Starlette, maybe it wouldn’t make sense fully, because the middleware has no idea about the routes that would handle the request, to extract the parameters. 🤔