Let Request.url_for lookup routes in mounted app first

starlette/requests.py

@@ -95,14 +95,23 @@ def url(self) -> URL:
         return self._url
 
     @property
-    def base_url(self) -> URL:
-        if not hasattr(self, "_base_url"):
+    def app_root_base_url(self) -> URL:
+        if not hasattr(self, "_app_root_base_url"):
             base_url_scope = dict(self.scope)
             base_url_scope["path"] = "/"
             base_url_scope["query_string"] = b""
             base_url_scope["root_path"] = base_url_scope.get(
                 "app_root_path", base_url_scope.get("root_path", "")
             )
+            self._app_root_base_url = URL(scope=base_url_scope)
+        return self._app_root_base_url
+
+    @property
+    def base_url(self) -> URL:
+        if not hasattr(self, "_base_url"):
+            base_url_scope = dict(self.scope)
+            base_url_scope["path"] = "/"
+            base_url_scope["query_string"] = b""
             self._base_url = URL(scope=base_url_scope)
         return self._base_url
 
@@ -170,9 +179,16 @@ def state(self) -> State:
         return self._state
 
     def url_for(self, name: str, **path_params: typing.Any) -> str:
-        router: Router = self.scope["router"]
-        url_path = router.url_path_for(name, **path_params)
-        return url_path.make_absolute_url(base_url=self.base_url)
+        app_root_router = self.scope["router"]
+        app_router = getattr(self.get('app', {}), 'router', None)
+        if app_router and app_router != app_root_router:
+            try: 
+                url_path = app_router.url_path_for(name, **path_params)
+                return url_path.make_absolute_url(base_url=self.base_url)
+            except Exception:
+                pass
+        url_path = app_root_router.url_path_for(name, **path_params)
+        return url_path.make_absolute_url(base_url=self.app_root_base_url)
 
 
 async def empty_receive() -> typing.NoReturn:

tests/test_routing.py

@@ -517,6 +517,45 @@ def test_url_for_with_root_path(test_client_factory):
     }
 
 
+
+def echo_urls_for(*keys):
+    def echo_urls(request):
+        def try_url_for(name):
+            try:
+                return request.url_for(name)
+            except NoMatchFound:
+                return 'NoMatchFound'
+        return JSONResponse({ k: try_url_for(k) for k in keys })
+    return echo_urls
+
+def test_url_with_sub_app(test_client_factory):
+    sub_app = Starlette(
+        routes=[
+            Route("/",  echo_urls_for('subapp_index','submount:subapp_index','index'), name="subapp_index", methods=["GET"])
+        ]
+    )
+    app = Starlette(
+        routes=[
+            Route("/", echo_urls_for('index', 'submount:subapp_index'), name="index", methods=["GET"]),
+            Mount('/submount', app=sub_app, name='submount')
+        ]
+    )
+
+    client = test_client_factory(
+        app, base_url="https://www.example.org/", root_path="/sub_path"
+    )
+    response = client.get("/")
+    assert response.json() == {
+        "index": "https://www.example.org/sub_path/",
+        "submount:subapp_index": "https://www.example.org/sub_path/submount/",
+    }
+    response = client.get("/submount/")
+    assert response.json() == {
+        "submount:subapp_index": "https://www.example.org/sub_path/submount/",
+        "subapp_index": "https://www.example.org/sub_path/submount/",
+        "index": "https://www.example.org/sub_path/",
+    }
+
 async def stub_app(scope, receive, send):
     pass  # pragma: no cover