@user_router.get("/")
def get_users():
    return [{"user_id": "u1"}, {"user_id": "u2"}]


@user_router.get("/{user_id}")
def get_user(user_id: str):
    return {"user_id": user_id}


@item_router.get("/")
def get_items(user_id: Optional[str] = None):
    if user_id is None:
        return [{"item_id": "i1", "user_id": "u1"}, {"item_id": "i2", "user_id": "u2"}]
    else:

                                },
                            }
                        },
                    },
                    "UserIn": {
                        "title": "UserIn",
                        "required": ["username", "email", "password"],
                        "type": "object",
                        "properties": {

                                "anyOf": [{"type": "string"}, {"type": "null"}],
                            },
                        },
                    },
                    "UserIn": {
                        "title": "UserIn",
                        "required": ["username", "password", "email"],
                        "type": "object",
                        "properties": {

* Allow path parameters to have default values (e.g. `None`) and discard them instead of raising an error.
    * This allows declaring a parameter like `user_id: str = None` that can be taken from a query parameter, but the same *path operation* can be included in a router with a path `/users/{user_id}`, in which case will be taken from the path and will be required.