class Item(BaseModel):
    id: int
    name: str


class ExceptionCapture:
    def __init__(self):
        self.exception = None

    def capture(self, exc):
        self.exception = exc
        return exc


app = FastAPI()
sub_app = FastAPI()
captured_exception = ExceptionCapture()

app.mount(path="/sub", app=sub_app)


@app.exception_handler(RequestValidationError)