Mounting ASGI subapps through FastAPI's router

fastapi_utils.py

import unittest.mock

from fastapi import FastAPI as _FastAPI, APIRouter as _APIRouter, routing

__all__ = "FastAPI", "APIRouter" 


class APIRouter(_APIRouter):
    def include_router(self, router, **kwargs) -> None:
        prefix = kwargs.get("prefix", "")
        super().include_router(router, **kwargs)
        # NOTE(zeronineseven): FastAPI does not handle ASGI subapps mounted to routers - they get simply ignored when
        #                      downstream router's routes get copied into the upstream. So this is a simple hack to
        #                      actually copy them too.
        for mount in (r for r in router.routes if isinstance(r, routing.Mount)):
            self.mount(prefix + mount.path, mount.app, mount.name)


class FastAPI(_FastAPI):
    def __init__(self, *args, **kwargs):
        called = False

        def create_router(*args_, **kwargs_):
            nonlocal called
            called = True
            return APIRouter(*args_, **kwargs_)

        with unittest.mock.patch("fastapi.routing.APIRouter", create_router):
            super().__init__(*args, **kwargs)

        if not called:
            raise AssertionError("Patching failed! Seems like something has changed in recent FastAPI versions...")

A (rather hack-ish) workaround for this problem.