@router.get("/router-depends/")
async def router_depends(commons: dict = Depends(common_parameters)):
    return {"in": "router-depends", "params": commons}


@router.get("/router-decorator-depends/", dependencies=[Depends(common_parameters)])
async def router_decorator_depends():
    return {"in": "router-decorator-depends"}


app.include_router(router)

client = TestClient(app)

    sub_router = APIRouter(lifespan=subrouter_lifespan)

    router = APIRouter(lifespan=router_lifespan)
    router.include_router(sub_router)

    app = FastAPI(lifespan=lifespan)
    app.include_router(router)

    @app.get("/")
    def main(request: Request) -> dict[str, str]:
        assert request.state.app
        assert request.state.router
        assert request.state.sub_router

We are importing the submodule `items` directly, instead of importing just its variable `router`.

This is because we also have another variable named `router` in the submodule `users`.

If we had imported one after the other, like:

```Python
from .routers.items import router
from .routers.users import router
```

import pytest
from fastapi import APIRouter


def test_router_circular_import():
    router = APIRouter()

    with pytest.raises(
        AssertionError,
        match="Cannot include the same APIRouter instance into itself. Did you mean to include a different router?",
    ):

@router_b.get("/")
def get_b():
    return {"msg": "B"}


@router_c.get("/")
def get_c():
    return {"msg": "C"}


router_b.include_router(router=router_c, prefix="/c")
router_a.include_router(router=router_b, prefix="/b")
app.include_router(router=router_a, prefix="/a")


client = TestClient(app)


@pytest.mark.parametrize(
    "path,expected_status,expected_response",
    [

        if responses is None:
            responses = {}
        for route in router.routes:
            if isinstance(route, APIRoute):
                combined_responses = {**responses, **route.responses}
                use_response_class = get_value_or_default(
                    route.response_class,
                    router.default_response_class,
                    default_response_class,

    response = client_nested.post("/outer/strict/items/", json={"key": "value"})
    assert response.status_code == 200


def test_default_inner_accepts_json_content_type():
    response = client_nested.post("/outer/default/items/", json={"key": "value"})
    assert response.status_code == 200


# Strict app -> lax outer router -> strict inner router

app_mixed = FastAPI(strict_content_type=True)

## Including Routers

When declaring routers, prefer to add router level parameters like prefix, tags, etc. to the router itself, instead of in `include_router()`.

Do this:

```python
from fastapi import APIRouter, FastAPI

app = FastAPI()

router = APIRouter(prefix="/items", tags=["items"])


@router.get("/")
async def list_items():
    return []

///

### Add the callback router { #add-the-callback-router }

At this point you have the *callback path operation(s)* needed (the one(s) that the *external developer*  should implement in the *external API*) in the callback router you created above.

Now use the parameter `callbacks` in *your API's path operation decorator* to pass the attribute `.routes` (that's actually just a `list` of routes/*path operations*) from that callback router:

## Custom `APIRoute` class in a router { #custom-apiroute-class-in-a-router }

You can also set the `route_class` parameter of an `APIRouter`:

{* ../../docs_src/custom_request_and_route/tutorial003_py310.py hl[26] *}