FAST002 rule produces false positives and incorrect suggestions

[MarkusSintonen]

Ruff (0.8.3) FAST002 produces false positives and suggests incorrect Annotated format with FastAPI params having default values. Following example can not be converted into the Annotated format as FastAPI does not support this format with params having default values. Also --unsafe-fixes fixes it to the format the FastAPI wont accept.

Ruff FAST002 violation:

def test_fast002_bug():
    from fastapi import FastAPI, Query
    app = FastAPI()

    @app.get("/test")
    def handler(echo: str = Query("")):  # FAST002 FastAPI dependency without `Annotated`
        return echo

But that is not compatible with FastAPI. It fails deeply with an AssertionError:

def test_fast002_bug():
    from typing import Annotated
    from fastapi import FastAPI, Query
    app = FastAPI()

    # AssertionError: `Query` default value cannot be set in `Annotated` for 'echo'. Set the default value with `=` instead.
    @app.get("/test")
    def handler(echo: Annotated[str, Query("")]):
        return echo

Used FastAPI 0.115.5.

The rule is pretty nice otherwise so thanks for it! Hopefully we get it fixed.

[MichaReiser]

Thanks for opening the issue. I think this is a true positive, but the fix is incorrect. The Query default value cannot be used inside Annotated according to FastAPI's documentation. Instead, a regular default value should be used. So the proper fix for your example is to rewrite it to

def test_fast002_bug():
    from typing import Annotated
    from fastapi import FastAPI, Query
    app = FastAPI()

    # AssertionError: `Query` default value cannot be set in `Annotated` for 'echo'. Set the default value with `=` instead.
    @app.get("/test")
    def handler(echo: Annotated[str, Query()] = ""):
        return echo