How to reroute requests to a different URL/endpoint in FastAPI?

Question

I am trying to write a middleware in my FastAPI application, so that requests coming to endpoints matching a particular format will be rerouted to a different URL, but I am unable to find a way to do that since request.url is read-only.

I am also looking for a way to update request headers before rerouting.

Are these things even possible in FastAPI?

Redirection is the best I could do so far:

from fastapi import Request
from fastapi.responses import RedirectResponse

@app.middleware("http")
async def redirect_middleware(request: Request, call_next):
    if matches_certain_format(request.url.path):
        new_url = create_target_url(request.url.path)
        return RedirectResponse(url=new_url)

Answer 1

To change the request's URL path—in other words, reroute the request to a different endpoint—one can simply modify the request.scope['path'] value inside the middleware, before processing the request, as demonstrated in Option 3 of this answer. If your API endpoints include path parameters (e.g., '/users/{user_id}'), then you mgiht want to have a look at this answer on how to extract that kind of path from the request object, and then compare it against a predefined list of routes_to_reroute, as shown below.

As for updating the request headers, or adding new custom headers to the request, you can follow a similar approach to the one described here, which demonstrates how to modify the request.scope['headers'] value.

Working Example

If you would like to avoid maintaining a list of routes to reroute and performing checks inside the middleware, you could instead mount a sub-application, which will contain only the routes that require rerouting, and add the middleware to that sub-app, similar to Option 3 of this answer.

from fastapi import FastAPI, Request

app = FastAPI()
routes_to_reroute = ['/']

@app.middleware('http')
async def some_middleware(request: Request, call_next):
    if request.url.path in routes_to_reroute:
        request.scope['path'] = '/welcome'
        headers = dict(request.scope['headers'])
        headers[b'custom-header'] = b'my custom header'
        request.scope['headers'] = [(k, v) for k, v in headers.items()]
        
    return await call_next(request)

@app.get('/')
async def main():
    return 'OK'

@app.get('/welcome')
async def welcome(request: Request):
    return {'msg': 'Welcome!', 'headers': request.headers}