fast-api-unused-path-parameter (FAST003)

Derived from the FastAPI linter.

Fix is sometimes available.

This rule is unstable and in preview. The --preview flag is required for use.

What it does

Identifies FastAPI routes that declare path parameters in the route path that are not included in the function signature.

Why is this bad?

Path parameters are used to extract values from the URL path.

If a path parameter is declared in the route path but not in the function signature, it will not be accessible in the function body, which is likely a mistake.

If a path parameter is declared in the route path, but as a positional-only argument in the function signature, it will also not be accessible in the function body, as FastAPI will not inject the parameter.

Known problems

If the path parameter is not a valid Python identifier (e.g., user-id, as opposed to user_id), FastAPI will normalize it. However, this rule simply ignores such path parameters, as FastAPI's normalization behavior is undocumented.

Example

from fastapi import FastAPI

app = FastAPI()


@app.get("/things/{thing_id}")
async def read_thing(query: str): ...

Use instead:

from fastapi import FastAPI

app = FastAPI()


@app.get("/things/{thing_id}")
async def read_thing(thing_id: int, query: str): ...

Fix safety

This rule's fix is marked as unsafe, as modifying a function signature can change the behavior of the code.