Dependencies, Sub-dependencies and Classes — FastAPI: From Basics to Production

Q: How do you require a check on every route in a router without changing each endpoint?

Pass `dependencies=[Depends(check)]` to `APIRouter(...)` (or `include_router`), so it runs for all routes; its return value isn't injected.

A dependency is any callable (function or class) that FastAPI calls before your endpoint, resolving its own parameters (query, headers, other dependencies). Dependencies form a graph: get_current_user depends on get_token which depends on a header. Classes work as dependencies too (Depends() with no argument uses the annotated class). Dependencies can be attached to routers or the whole app for cross-cutting checks.

from typing import Annotated
from fastapi import APIRouter, Depends, Header, HTTPException


async def get_token(authorization: Annotated[str | None, Header()] = None) -> str:
    if not authorization or not authorization.startswith("Bearer "):
        raise HTTPException(401, "Missing bearer token", headers={"WWW-Authenticate": "Bearer"})
    return authorization.removeprefix("Bearer ")


async def get_current_user(token: Annotated[str, Depends(get_token)], db: DB) -> User:
    return await auth.user_from_token(db, token)


CurrentUser = Annotated[User, Depends(get_current_user)]


async def require_verified(user: CurrentUser) -> None:
    if not user.email_verified:
        raise HTTPException(403, "Verify your email first")


router = APIRouter(prefix="/billing", dependencies=[Depends(require_verified)])

Common pitfalls

Business logic inside dependencies that should live in services.
Raising generic exceptions instead of HTTPException in auth dependencies.
Forgetting dependencies are cached per request (usually desirable).

Key points

Dependencies form a graph
Router/app-level dependencies for cross-cutting checks
Annotated aliases like CurrentUser

Further reading: FastAPI: Sub-dependencies

Practice questions

How do you require a check on every route in a router without changing each endpoint?

Answer

Pass dependencies=[Depends(check)] to APIRouter(...) (or include_router), so it runs for all routes; its return value isn't injected.

Can a class be a dependency?

Answer

Yes. FastAPI calls the class like a function, resolving its __init__ parameters, and injects the instance — useful for parameter groups like pagination.