How to re-route requests to a different URL/endpoint in FastAPI?

I am trying to write a middleware in my FastAPI application, so that requests coming to endpoints matching a particular format will be re-routed 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 re-routing.

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

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)


  • To change the request's URL path—in other words, re-route 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 pre-defined 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 re-route and performing checks inside the middleware, you could instead mount a sub-application, which will contain only the routes that require re-routing, 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 = ['/']
    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)
    async def main():
        return 'OK'
    async def welcome(request: Request):
        return {'msg': 'Welcome!', 'headers': request.headers}