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Graph/parameterized-dependency-closure

Pattern

Parameterized Dependency via Closure

parameterized-dependency-closure

A recurring FastAPI shape where request-time parameters (e.g., auth tokens) can’t be passed directly into Depends/dependencies, so you return a callable (closure/factory) that captures arguments and is used as the dependency.

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Updated4/30/2026

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29 linked nodes

Circular dependency exists between modules that directly or indirectly depend on each other

Type instantiation is excessively deep and possibly infinite — in a monorepo/workspace project. Tension: on seemingly benign types like this. Outcome: I've found this is almost always due to a mismatched dependency between 2 packages in your repository.

Outcome: A solution for me was to add some dependencies.

Pydantic does not support scipy types — make a web api endpoint, using the python package FastAPI. Tension: I do not want to convert to some dense representation, this is wasteful.

I have an endpoint that takes a Pydantic model, `Foo`, as a query parameter. Outcome: parsing fails even without nesting.

Is there a way to pass an argument to a function defined within the `dependencies` argument — within the `dependencies` argument of, for instance, `app.get`. Tension: I really want to do is provide an argument to `verify_token`. Outcome: there isn't, what other options exist ?

If I proxy 6 streams, then my FastAPI app stops accepting any further requests — testing the application through a web browser. Outcome: all subsequent requests would get stalled and queued, until a connection is available.

Is there any "Depends" equivalent in FastHTML? — In "fastapi" you can use "Depends" in a endpoint. Tension: fastapi and fastHTML are focusing on diffrent logics. Outcome: it is not a feature on fastHTML.

there's no way to set auth tokens in the docs — implement a basic `Bearer` Authorization in FastAPI.

FastAPI route dependency injection with a Beanie `Document` type fails with an error indicating initialized collections are not detected when `Product` is used as a parameter in endpoint functions across routers.

Waiting for background tasks to complete. — Whenever I gracefully shut down my FastAPI app by saving a file (with the `--reload` option). Tension: "Shutting down sessions" is never logged because it is waiting for all background tasks to be completed, which will never happen. Outcome: How am I supposed to close my sessions so that FastAPI can properly shut down?

decoded_token = keycloak_openid.decode_token( token,validate=True, ) — I am trying to use keycloak in my FastAPI app. Tension: verify_token(token: str = Depends(oauth2_scheme)).

How to serve a machine-learning model via FastAPI using dependencies when the model must be held in a stateful class instance that is updated on a schedule, rather than recreated per request.

trying to perform data processing during the application's startup. Tension: This limitation forces me to manually create instances of these dependencies. Outcome: this approach doesn't work as expected.

FastAPI returns a 422 Unprocessable Entity error for a UUID path parameter when building the parameter as a modular dependency using `Path(alias=...)`.

Protect FastAPI routes so that authenticated users without the required permissions/roles cannot access specific endpoints.

using FastAPI. Outcome: If I do: `@app.post("/instance/new", tags=["instance"], PlainTextResponse)` I get an error when returning a JSON response.

When deployed behind a Kubernetes/API-gateway URL prefix (e.g., https://api-gateway.example.com/myapp), FastAPI tried to load OpenAPI from a path missing the prefix (e.g., /api/v1/openapi.json), causing a 404 and failing to load API definitions.

Running pytest tests with FastAPI triggers a DeprecationWarning from httpx: the 'app' shortcut is deprecated and should use explicit transport configuration. The warning occurs when creating TestClient in a fixture.

In a FastAPI endpoint that accepts a Pydantic model, the caller can supply all model fields (including `id`), but the user wants to allow only `name` to be provided while keeping `id` server-generated. Attempting to use `Body(include=['id'])`/type-level inclusion breaks FastAPI/Pydantic validation and error handling behavior.

objects seem to be assumed to arrive in the body in `fastapi` — I'm trying to receive a nontrivial query parameter object. Tension: I've had to do something like this on the server. Outcome: it would be nice for `fastapi` to do this for me directly in the function declaration.

I have had this brain teaser for a day now — map certain models to build an API (Learning Management System API) using FastAPI. Outcome: I pretty much just used BaseRole as a Mixin where I need it to be.

Middleware registered on the main FastAPI app is also triggered for requests routed to a mounted sub-application, including subapi docs/openapi endpoints. This prevents isolating middleware behavior per sub-application.

"type":"missing","loc":["query","name"]. Tension: The query will return None and FastApi will fail to validate that parameter is a Person object. Outcome: Any post request to the endpoint /post_with_dependency will indeed return a 422 Unprocessable Entity error.

I need to upload it to an app, which does not support schemas that have been defined using `anyOf`, `allOf` or `OneOf`. — openapi.json, which has been automatically created by FastAPI (version 0.109.0). Outcome: It looks like the error is coming from the third `parameter` definition and the use of `allOf`.

Defining FastAPI routes in a loop causes each endpoint to behave like the last model

The async thunk payloadCreator is defined with two parameters because the user wants no input payload but needs rejectWithValue, and they are unsure about the correct way to handle a callback with no parameters.

Is there an equivalent in FastAPI, SQLAlchemy or Pydantic? — I have a Django project running on production with PostgreSQL. Tension: I do not want to rewrite all the models in FastAPI using pydantic, and at the same time I don't want to make a mistake.

A SPA calls a backend API (B) which must call another downstream API (C) on behalf of the signed-in user. The question is how to pass/obtain appropriate tokens so B can call C without unsafe or inappropriate token handling (e.g., forwarding an identity token).

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