Make FastAPI's root_path take precedence

fastapi/applications.py

@@ -1052,12 +1052,12 @@ class FastAPI(Starlette):
         if self.root_path:
             root_path = scope.get("root_path", "")
             if root_path and self.root_path != root_path:
-                raise RuntimeError(
+                logger.warning(
                     f"The ASGI server is using a different root path than the one "
                     f"configured in FastAPI. The configured root path is: "
                     f"{self.root_path}, the ASGI server root path is: {root_path}. "
+                    f"The former will be used."
                 )
-            else:
             scope["root_path"] = self.root_path
             path = scope.get("path")
             if path and not path.startswith(self.root_path):

tests/test_root_path_redirects.py

@@ -34,3 +34,19 @@ def test_redirects_with_root_path():
     response = client.get("/hello", follow_redirects=False)
     assert response.status_code == 307
     assert response.headers["location"] == "http://testserver/api/hello/"
+
+
+def test_invalid_combination_of_root_path():
+    app = FastAPI(root_path="/api")
+    router = APIRouter()
+
+    @router.get("/hello/")
+    def hello_page() -> str:
+        return "Hello, World!"
+
+    app.include_router(router)
+
+    client = TestClient(app, base_url="http://testserver", root_path="/notapi")
+    response = client.get("/hello", follow_redirects=False)
+    assert response.status_code == 307
+    assert response.headers["location"] == "http://testserver/api/hello/"