FastAPI Depends() DI flagged by Flake8 Bugbear Linting

[shawnwall]

Flake8 Bugbear is a linting tool that points out potential bugs and design problems in applications. While doing linting on my FastAPI project, bugbear calls out:

B008 Do not perform function calls in argument defaults. The call is performed only once at function definition time. All calls to your function will reuse the result of that definition-time function call. If this is intended, assign the function call to a module-level variable and use that variable as a default value.

... for any Depends() usage in a method signature, such as:

async def login(response: Response,
                request: Request,
                form_data:
                OAuth2PasswordRequestForm = Depends())

I'm not flagging this as a bug, but just wanted to raise it to get your design opinion, as apparently someone out there thinks the Depends() pattern is not very pythonic.

[phy25]

It is felt that these lints don’t belong in the main Python tools as they are very opinionated and do not have a PEP or standard behind them. Due to flake8 being designed to be extensible, the original creators of these lints believed that a plugin was the best route. This has resulted in better development velocity for contributors and adaptive deployment for flake8 users.

https://pypi.org/project/flake8-bugbear/

[tiangolo]

Thanks for the help here @phy25 ! 👏 🙇

Yep, I think that should answer the question ✔️

[github-actions]

Assuming the original issue was solved, it will be automatically closed now. But feel free to add more comments or create new issues.

[ghost]

For those who want to keep bugbear but not disable the checks entirely for every line that uses Depends, the following small plugin will amend bugbear's rules to recognize Depends as immutable, Unfortunately, BugBear does not provide a simpler configuration process.

from typing import Any

import bugbear

class Extension(object):
    name = "bugbear_fastapi_depends"
    version = 1

    def __init__(self, noqa: Any) -> None:
        pass

bugbear.B008.immutable_calls.update([
    "fastapi.Depends",
    "fastapi.params.Depends",
    "Depends",
])

For local usage, from setup.cfg, you can do:

[flake8:local-plugins]
extension =
     BB8F = module.of.my.extension:Extension

[ShaneNolan]

Anyone using a setup.cfg flake8 configuration can use the extend-immutable-calls keyword to allow Depends().

[flake8]
...
extend-immutable-calls = fastapi.Depends, fastapi.params.Depends

[phouverneyuff]

Using ruff, I had set this on pyproject.toml

[tool.ruff.flake8-bugbear]
extend-immutable-calls = ["fastapi.Depends", "fastapi.params.Depends", "fastapi.Query", "fastapi.params.Query"]