routes_relative.py

class ApiRoot:
    Base = "/api"
    Frontend = "/ui"

class Users:
    inner_base = "/users"

    Register = inner_base + "/register"

    ByName = lambda name: ApiRoot.Users.inner_base + f"/{name}" 

fastapi_app = FastAPI()

users_router = APIRouter(prefix=Users.inner_base)

@users_router.post(Users.Register)
async def users_register():
    return None

@users_router.get(Users.ByName("{name}"))
async def users_get_by_name(name: str):
    return name

fastapi_app.include_router(users_router, prefix=ApiRoot.Base)