Add `routes` argument to Mount (#400)

* Add Mount(routes=...)

* Tweaks

scripts/lint

@@ -7,6 +7,7 @@ fi
 
 set -x
 
+${PREFIX}mypy starlette --ignore-missing-imports --disallow-untyped-defs
 ${PREFIX}autoflake --in-place --recursive starlette tests
 ${PREFIX}black starlette tests
 ${PREFIX}isort --multi-line=3 --trailing-comma --force-grid-wrap=0 --combine-as --line-width 88 --recursive --apply starlette tests

starlette/routing.py

@@ -276,10 +276,23 @@ class WebSocketRoute(BaseRoute):
 
 
 class Mount(BaseRoute):
-    def __init__(self, path: str, app: ASGIApp, name: str = None) -> None:
+    def __init__(
+        self,
+        path: str,
+        app: ASGIApp = None,
+        routes: typing.List[BaseRoute] = None,
+        name: str = None,
+    ) -> None:
         assert path == "" or path.startswith("/"), "Routed paths must start with '/'"
+        assert (
+            app is not None or routes is not None
+        ), "Either 'app', or 'routes' must be specified"
         self.path = path.rstrip("/")
-        self.app = app
+        if routes is None:
+            assert app is not None
+            self.app = app
+        else:
+            self.app = Router(routes=routes)
         self.name = name
         self.path_regex, self.path_format, self.param_convertors = compile_path(
             path + "/{path:path}"

tests/test_routing.py

@@ -38,14 +38,12 @@ app = Router(
         Route("/", endpoint=homepage, methods=["GET"]),
         Mount(
             "/users",
-            app=Router(
-                [
-                    Route("/", endpoint=users),
-                    Route("/me", endpoint=user_me),
-                    Route("/{username}", endpoint=user),
-                    Route("/nomatch", endpoint=user_no_match),
-                ]
-            ),
+            routes=[
+                Route("/", endpoint=users),
+                Route("/me", endpoint=user_me),
+                Route("/{username}", endpoint=user),
+                Route("/nomatch", endpoint=user_no_match),
+            ],
         ),
         Mount("/static", app=staticfiles),
     ]