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Graph/fastapi-dependency-callable-misuse

AntiPattern

FastAPI Dependency Callable Misuse

fastapi-dependency-callable-misuse

FastAPI dependency injection fails or behaves inconsistently when dependencies are implemented as the wrong kind of callable (e.g., decorated async functions, router-wide models, or dependency factories) and type/initialization expectations don’t match, breaking routes across modules.

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Updated8/1/2026

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30 linked nodes

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Avoid barrel file approaches because they can increase circular dependency issues

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Yes, it's Ok to use such decorators. — @review_router.get("/review") @admin_required async def get_review():. Tension: This use-case is covered by tests in FastAPI repo. Outcome: This use-case is covered by tests in FastAPI repo: https://github.com/fastapi/fastapi/blob/6513d4daa16a536d17743de6a292f49bd06388a4/tests/test_dependency_wrapped.py#L183-L200.

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Pass `BackgroundTasks` from the route into your dependency. — when declared inside an async dependency. Tension: the background job never runs. Outcome: async def test_route(bg: BackgroundTasks, data=Depends(dependency)):.

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If you convert all operations to perform asynchronously (using `aiosqlite`) , the issue will be mitigated. — When a route in FastAPI is listed as asynchronous (`async def`). Tension: that doesn't fully solve the inherent single writer problem of Sqlite. Outcome: Here's a fully operational update of the FastAPI server logic:.

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Removing `async` before the function definition solved it. — class LoggingAPIRoute(APIRoute):. Outcome: def get_route_handler(self) -> Callable:.

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The easy way is to have the function return another function — `verify_token("red")` returns a callable with the right syntax and type annotations. Tension: FastAPI can use it as a dependency.

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This issue on github offers a solution github. — since fastapi and fastHTML are focusing on diffrent logics. Tension: there are diffrent tools included. Outcome: but since fastapi and fastHTML are focusing on diffrent logics.

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Move the initialization of `qdrant_client` into lifespan. Outcome: override using standard FastAPI mechanism.

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You can use a global dependency: — FastAPI. Tension: there's no way to set auth tokens in the docs. Outcome: app.openapi = custom_openapi.

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Change the engine async fixture from session scope to function scope (remove scope="session") so all async fixtures/tests run on the same event loop, preventing the cross-loop Future/Task error.

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Try `fastapi run` instead of `fastapi dev` — if you use --reload or reload=True.

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Create a lifespan-managed shared state (e.g., instantiate a state manager/model in FastAPI lifespan) and store the loaded model in that state so request handlers can access it via Request/app state or a dependency that reads from lifespan.

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using `magicmock` to mock it could lead to unusual or unexpected behavior when checking for call counts — Since `get_item` might be an asynchronous function in your middleware. Tension: using `magicmock` to mock it could lead to unusual or unexpected behavior. Outcome: which is designed to work with async functions.

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This mismatch necessitates a manual conversion of the OpenAPI document to version 2.0 — before it can be utilized by API Gateway. Tension: Incompatibility issue between the OpenAPI document supported by GCP API Gateway, and the OpenAPI document automatically generated from FastAPI itself. Outcome: The Solution: For this will be clearly explained in the Link.

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Implement role-based authorization as a reusable dependency (e.g., a callable class used with Depends) that loads the current user from the token and raises HTTP 403 when the user's role is not in the allowed roles; apply the dependency to routes.

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use `pytest_asyncio` and the async fixture — If you don't want to utilize an `AsyncClient` like the one by `httpx`. Tension: ensuring that the FastAPI app's lifespan context correctly works.

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Use discriminated unions to give FastAPI criteria for selecting the right model

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Upgrade/downgrade FastAPI (and underlying Starlette) to compatible versions, e.g., reinstall with a specific FastAPI version such as fastapi==0.115.6 after uninstalling fastapi/starlette/pydantic.

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or use Depends() with Pydantic model — from fastapi import APIRouter. Outcome: async def add_job(job: Job = Depends()):.

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You can use multiple decorators for the same function — FastAPI. Outcome: return [{"get": "sql_alchemy_output_from_{master_i}_here"}].

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I have solved it using the lifespan of FastAPI. — use it inside the routes. Tension: I couldn't identify where the problem is. Outcome: I have just initialized the Pubsub client in LifeSpan.

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Do not rely on parent-app `@app.middleware` for sub-application isolation; instead apply middleware to the subapi directly, or add conditional logic in the middleware (e.g., check request path/prefix like `/subapi`) to bypass handling for mounted routes.

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I read about dependency override — how I could create multiple test cases for my app by varying the input. Tension: I am not sure if that is mandatory.

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Attach an authentication dependency to only the endpoints that must be protected—either by adding `Depends(...)` (e.g., `HTTPBasic` or the configured AuthX dependency) directly on each protected route or by registering those routes under a separate `APIRouter` with the required auth dependency.

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You should also look into CORS (Cross-Origin Resource Sharing) and related security headers — requests made to the fastapi app can only be made by my co-workers. Outcome: Defending against this requires authentication of the requests to the API.

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case "CallbackRouteError": return { error: "Invalid credentials!" } — in my `login.ts` server action in the switch case clause. Tension: to return the correct errors. Outcome: bypass this error.

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Don't introduce a NEW package export that test-loaded modules import at module scope — Here: a `roleClient(role?)` helper in the consumer package (`etl/llm-route.ts`). Tension: it forces you to update every `vi.mock` factory, and missing one re-breaks CI.

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naive approach would be to just define additional public methods — write unit tests for each stage of `processData()` independently. Tension: the common interface required for polymorphic behavior. Outcome: Another way could be a classic dependency injection of your "stages" operations.

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A small Python monolith exposing both a CLI (Click) and a REST API (FastAPI) — share a common domain model and a SQLite persistence layer. Outcome: share a common domain model and a SQLite persistence layer.

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Don't build per-client adapters. — client capabilities change with updates. Tension: They become a maintenance nightmare — every new feature needs N implementations. Outcome: one delivery function that degrades.

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