🐛 Fix mount subapp with APIrouter

fastapi/routing.py

@@ -778,11 +778,19 @@ class APIRouter(routing.Router):
                 self.add_websocket_route(
                     prefix + route.path, route.endpoint, name=route.name
                 )
+            elif isinstance(route, routing.Mount):
+                self.mount(prefix + route.path, route.app, name=route.name)
         for handler in router.on_startup:
             self.add_event_handler("startup", handler)
         for handler in router.on_shutdown:
             self.add_event_handler("shutdown", handler)
 
+    def mount(self, path: str, app: ASGIApp, name: Optional[str] = None) -> None:
+        route = routing.Mount(path, app=app, name=name)
+        if route.__getattribute__("path") == "":
+            route.__setattr__("path", "/")
+        self.routes.append(route)
+
     def get(
         self,
         path: str,

tests/test_mount_app.py

@@ -0,0 +1,67 @@
+from fastapi import APIRouter, FastAPI
+from fastapi.testclient import TestClient
+
+app = FastAPI()
+app2 = FastAPI()
+app3 = FastAPI()
+app4 = FastAPI()
+
+
+@app.get("/a")
+async def a():
+    return "a"
+
+
+@app2.get("/b")
+async def b():
+    return "b"
+
+
+app.mount("/2", app2)
+
+
+@app3.get("/c")
+def c():
+    return "c"
+
+
+router = APIRouter()
+router.mount("/3", app3)
+app.include_router(router)
+
+
+@app4.get("/")
+def d():
+    return "d"
+
+
+router2 = APIRouter()
+router2.mount("/", app4)
+app.include_router(router2)
+
+client = TestClient(app)
+
+
+def test_a():
+    response = client.get("/a")
+    assert response.status_code == 200, response.text
+    assert response.json() == "a"
+
+
+def test_b():
+    response = client.get("/2/b")
+    assert response.status_code == 200, response.text
+    assert response.json() == "b"
+
+
+def test_c():
+    response = client.get("/3/c")
+    assert response.status_code == 200, response.text
+    assert response.json() == "c"
+
+
+def test_mount():
+    response = client.get("/")
+    assert response.status_code == 200, response.text
+    assert response.json() == "d"
+    assert router2.routes[0].path == "/"