Add ability to override default root APIRouter class

uriyyo · uriyyo · commit b965e33430af · 2025-05-04T11:50:44.000+02:00
diff --git a/fastapi/applications.py b/fastapi/applications.py
@@ -810,6 +810,14 @@ class Item(BaseModel):
                 """
             ),
         ] = True,
+        router_class: Annotated[
+            Type[routing.APIRouter],
+            Doc(
+                """
+                Custom router class to be used by this application.
+                """,
+            ),
+        ] = routing.APIRouter,
         **extra: Annotated[
             Any,
             Doc(
@@ -929,7 +937,7 @@ class Item(BaseModel):
                 """
             ),
         ] = {}
-        self.router: routing.APIRouter = routing.APIRouter(
+        self.router: routing.APIRouter = router_class(
             routes=routes,
             redirect_slashes=redirect_slashes,
             dependency_overrides_provider=self,
diff --git a/tests/test_custom_router_class.py b/tests/test_custom_router_class.py
@@ -0,0 +1,40 @@
+from fastapi import APIRouter, FastAPI, status
+from fastapi.testclient import TestClient
+from starlette.datastructures import MutableHeaders
+
+
+class CustomAPIRouter(APIRouter):
+    async def __call__(self, scope, receive, send):
+        async def send_wrapper(message):
+            if message["type"] == "http.response.start":
+                headers = MutableHeaders(scope=message)
+                headers["x-custom-header"] = "custom-value"
+
+            await send(message)
+
+        await super().__call__(scope, receive, send_wrapper)
+
+
+app = FastAPI(
+    router_class=CustomAPIRouter,
+)
+
+
+@app.get("/")
+def get_root():
+    return {"message": "root"}
+
+
+client = TestClient(app)
+
+
+def test_get():
+    response = client.get("/")
+
+    assert response.status_code == status.HTTP_200_OK
+    assert response.json() == {"message": "root"}
+    assert response.headers["x-custom-header"] == "custom-value"
+
+
+def test_route_classes():
+    assert isinstance(app.router, CustomAPIRouter)
