Support sub-apps

fastapi/routing.py

@@ -360,8 +360,11 @@ def get_request_handler(
             if dependant.call
             else EndpointContext()
         )
+
         if dependant.path:
-            endpoint_ctx["path"] = f"{request.method} {dependant.path}"
+            # For mounted sub-apps, include the mount path prefix
+            mount_path = request.scope.get("root_path", "").rstrip("/")
+            endpoint_ctx["path"] = f"{request.method} {mount_path}{dependant.path}"
 
         # Read body and auto-close files
         try:
@@ -491,8 +494,9 @@ def get_websocket_app(
             else EndpointContext()
         )
         if dependant.path:
-            endpoint_ctx["path"] = f"WS {dependant.path}"
-
+            # For mounted sub-apps, include the mount path prefix
+            mount_path = websocket.scope.get("root_path", "").rstrip("/")
+            endpoint_ctx["path"] = f"WS {mount_path}{dependant.path}"
         async_exit_stack = websocket.scope.get("fastapi_inner_astack")
         assert isinstance(async_exit_stack, AsyncExitStack), (
             "fastapi_inner_astack not found in request scope"

tests/test_validation_error_context.py

@@ -23,16 +23,28 @@ class ExceptionCapture:
 
 
 app = FastAPI()
+sub_app = FastAPI()
 captured_exception = ExceptionCapture()
 
+app.mount(path="/sub", app=sub_app)
+
 
 @app.exception_handler(RequestValidationError)
+@sub_app.exception_handler(RequestValidationError)
 async def request_validation_handler(request: Request, exc: RequestValidationError):
     captured_exception.capture(exc)
     raise exc
 
 
+@app.exception_handler(ResponseValidationError)
+@sub_app.exception_handler(ResponseValidationError)
+async def response_validation_handler(_: Request, exc: ResponseValidationError):
+    captured_exception.capture(exc)
+    raise exc
+
+
 @app.exception_handler(WebSocketRequestValidationError)
+@sub_app.exception_handler(WebSocketRequestValidationError)
 async def websocket_validation_handler(
     websocket: WebSocket, exc: WebSocketRequestValidationError
 ):
@@ -50,6 +62,11 @@ def get_item():
     return {"name": "Widget"}
 
 
+@sub_app.get("/items/", response_model=Item)
+def get_sub_item():
+    return {"name": "Widget"}  # pragma: no cover
+
+
 @app.websocket("/ws/{item_id}")
 async def websocket_endpoint(websocket: WebSocket, item_id: int):
     await websocket.accept()  # pragma: no cover
@@ -57,6 +74,13 @@ async def websocket_endpoint(websocket: WebSocket, item_id: int):
     await websocket.close()  # pragma: no cover
 
 
+@sub_app.websocket("/ws/{item_id}")
+async def subapp_websocket_endpoint(websocket: WebSocket, item_id: int):
+    await websocket.accept()  # pragma: no cover
+    await websocket.send_text(f"Item: {item_id}")  # pragma: no cover
+    await websocket.close()  # pragma: no cover
+
+
 client = TestClient(app)
 
 
@@ -95,6 +119,33 @@ def test_websocket_validation_error_includes_endpoint_context():
     assert "/ws/" in error_str
 
 
+def test_subapp_request_validation_error_includes_endpoint_context():
+    captured_exception.exception = None
+    try:
+        client.get("/sub/items/")
+    except Exception:
+        pass
+
+    assert captured_exception.exception is not None
+    error_str = str(captured_exception.exception)
+    assert "get_sub_item" in error_str
+    assert "/sub/items/" in error_str
+
+
+def test_subapp_websocket_validation_error_includes_endpoint_context():
+    captured_exception.exception = None
+    try:
+        with client.websocket_connect("/sub/ws/invalid"):
+            pass  # pragma: no cover
+    except Exception:
+        pass
+
+    assert captured_exception.exception is not None
+    error_str = str(captured_exception.exception)
+    assert "subapp_websocket_endpoint" in error_str
+    assert "/sub/ws/" in error_str
+
+
 def test_validation_error_with_only_path():
     errors = [{"type": "missing", "loc": ("body", "name"), "msg": "Field required"}]
     exc = RequestValidationError(errors, endpoint_ctx={"path": "GET /api/test"})