Add support for base url

[ArtyomVancyan]

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Example Code

from fastapi import FastAPI, Request
from starlette.middleware.base import BaseHTTPMiddleware
from starlette.responses import Response


class URLPrefixMiddleware(BaseHTTPMiddleware):
    def __init__(self, app: FastAPI, prefix: str = "/fastapi-oauth2"):
        super().__init__(app)
        self.prefix = prefix

    async def dispatch(self, request: Request, call_next):
        response: Response = await call_next(request)
        # Modify the Location header if it's a redirect response
        if response.status_code in (301, 302, 303, 307, 308) and "location" in response.headers:
            location = response.headers["location"]
            # Ensure the location is not already prefixed
            if not location.startswith(self.prefix):
                response.headers["location"] = self.prefix + location
        return response


def prefixify(app: FastAPI, prefix: str = "/fastapi-oauth2") -> FastAPI:
    root = FastAPI()
    root.mount(prefix, app)
    return root


app = FastAPI()
app = prefixify(app)

Description

What I need is the opposite of FastAPI(root_path="/some/path"). The above code simulates my expected behavior, but I have multiple applications that I want to deploy under the same URL origin and access each of them using their specific prefix paths without changing anything in them. This is a rare but actual problem, and FastAPI must have this feature. See how VitePress supports it.

Just to clarify, the .mount method almost does what I need, but if an application has redirect responses, it doesn't work as expected. That is why I created the URLPrefixMiddleware to handle that case too.

A most elegant way of solving this problem, in my personal opinion, would be adding an argument to the .mount method so it handles the response URLs, too. I WILL CONTRIBUTE, YOU JUST APPROVE THE IDEA.

Operating System

Linux

FastAPI Version

0.110.0

Pydantic Version

2.4.2

Python Version

3.10.12